[REGRESSION][BISECTED] Commit 60e3318e3e900 in stable/linux-6.1.y breaks cifs client failover to another server in DFS namespace

[Andrew Paniakin <apanyaki@amazon.com>]

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Commit 60e3318e3e900 ("cifs: use fs_context for automounts") was
released in v6.1.54 and broke the failover when one of the servers
inside DFS becomes unavailable. We reproduced the problem on the EC2
instances of different types. Reverting aforementioned commint on top of
the latest stable verison v6.1.94 helps to resolve the problem.

Earliest working version is v6.2-rc1. There were two big merges of CIFS fixes:
[1] and [2]. We would like to ask for the help to investigate this problem and
if some of those patches need to be backported. Also, is it safe to just revert
problematic commit until proper fixes/backports will be available?

We will help to do testing and confirm if fix works, but let me also list the
steps we used to reproduce the problem if it will help to identify the problem:
1. Create Active Directory domain eg. 'corp.fsxtest.local' in AWS Directory
Service with:
- three AWS FSX file systems filesystem1..filesystem3
- three Windows servers; They have DFS installed as per
  https://learn.microsoft.com/en-us/windows-server/storage/dfs-namespaces/dfs-overview:
    - dfs-srv1: EC2AMAZ-2EGTM59
    - dfs-srv2: EC2AMAZ-1N36PRD
    - dfs-srv3: EC2AMAZ-0PAUH2U 

 2. Create DFS namespace eg. 'dfs-namespace' in Windows server 2008 mode
 and three folders targets in it:
- referral-a mapped to filesystem1.corp.local
- referral-b mapped to filesystem2.corp.local
- referral-c mapped to filesystem3.corp.local
- local folders dfs-srv1..dfs-srv3 in C:\DFSRoots\dfs-namespace of every
  Windows server. This helps to quickly define underlying server when
  DFS is mounted.

3. Enabled cifs debug logs:
```
echo 'module cifs +p' > /sys/kernel/debug/dynamic_debug/control
echo 'file fs/cifs/* +p' > /sys/kernel/debug/dynamic_debug/control
echo 7 > /proc/fs/cifs/cifsFYI
```

4. Mount DFS namespace on Amazon Linux 2023 instance running any vanilla
kernel v6.1.54+:
```
dmesg -c &>/dev/null
cd /mnt
mount -t cifs -o cred=/mnt/creds,echo_interval=5 \
    //corp.fsxtest.local/dfs-namespace \
    ./dfs-namespace
```

5. List DFS root, it's also required to avoid recursive mounts that happen
during regular 'ls' run:
```
sh -c 'ls dfs-namespace'
dfs-srv2  referral-a  referral-b
```

The DFS server is EC2AMAZ-1N36PRD, it's also listed in mount:
```
[root@ip-172-31-2-82 mnt]# mount | grep dfs
//corp.fsxtest.local/dfs-namespace on /mnt/dfs-namespace type cifs (rw,relatime,vers=3.1.1,cache=strict,username=Admin,domain=corp.fsxtest.local,uid=0,noforceuid,gid=0,noforcegid,addr=172.31.11.26,file_mode=0755,dir_mode=0755,soft,nounix,mapposix,rsize=4194304,wsize=4194304,bsize=1048576,echo_interval=5,actimeo=1,closetimeo=1)
//EC2AMAZ-1N36PRD.corp.fsxtest.local/dfs-namespace/referral-a on /mnt/dfs-namespace/referral-a type cifs (rw,relatime,vers=3.1.1,cache=strict,username=Admin,domain=corp.fsxtest.local,uid=0,noforceuid,gid=0,noforcegid,addr=172.31.12.80,file_mode=0755,dir_mode=0755,soft,nounix,mapposix,rsize=4194304,wsize=4194304,bsize=1048576,echo_interval=5,actimeo=1,closetimeo=1)
```

List files in first folder:
```
sh -c 'ls dfs-namespace/referral-a'
filea.txt.txt
```

6. Shutdown DFS server-2.
List DFS root again, server changed from dfs-srv2 to dfs-srv1 EC2AMAZ-2EGTM59:
```
sh -c 'ls dfs-namespace'
dfs-srv1  referral-a  referral-b
```

7. Try to list files in another folder, this causes ls to fail with error:
```
sh -c 'ls dfs-namespace/referral-b'
ls: cannot access 'dfs-namespace/referral-b': No route to host```

Sometimes it's also 'Operation now in progress' error.

mount shows the same output:
```
//corp.fsxtest.local/dfs-namespace on /mnt/dfs-namespace type cifs (rw,relatime,vers=3.1.1,cache=strict,username=Admin,domain=corp.fsxtest.local,uid=0,noforceuid,gid=0,noforcegid,addr=172.31.11.26,file_mode=0755,dir_mode=0755,soft,nounix,mapposix,rsize=4194304,wsize=4194304,bsize=1048576,echo_interval=5,actimeo=1,closetimeo=1)
//EC2AMAZ-1N36PRD.corp.fsxtest.local/dfs-namespace/referral-a on /mnt/dfs-namespace/referral-a type cifs (rw,relatime,vers=3.1.1,cache=strict,username=Admin,domain=corp.fsxtest.local,uid=0,noforceuid,gid=0,noforcegid,addr=172.31.12.80,file_mode=0755,dir_mode=0755,soft,nounix,mapposix,rsize=4194304,wsize=4194304,bsize=1048576,echo_interval=5,actimeo=1,closetimeo=1)
```

I also attached kernel debug logs from this test.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=851f657a86421
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0a924817d2ed9

Reported-by: Andrei Paniakin <apanyaki@amazon.com>
Bisected-by: Simba Bonga <simbarb@amazon.com>
---

#regzbot introduced: v6.1.54..v6.2-rc1

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