getxattr() on cifs sometimes hangs since kernel 5.14

[Forest <forestix@sonic.net>]

When running on recent kernel versions, this system call on a cifs-mounted
file sometimes takes an unusually long time:

getxattr("/cifsmount/dir/image.jpg", "user.baloo.rating", NULL, 0)

The call normally returns in under 10 milliseconds, but on kernel 5.14+, it
sometimes takes over 30 seconds with no significant client or server load.

Discovered while using gwenview to browse 100+ 1.5 MiB images on a samba share
mounted via /etc/fstab. While quickly flipping through the images, the problem
often occurs within 20 seconds. Gwenview freezes until the call completes.

Client:
  kernel versions 5.14 and later
  mount.cifs 6.11
  Gwenview 20.12.3
  Debian Bullseye
  4-core amd64
Server:
  Samba 4.13.13-Debian
  Debian Bullseye
  6-core arm64 

A git bisect identified kernel commit 9e992755be8f as the problematic change.
The problem does not occur when any of the following are true:
- Client is running a kernel from before that commit.
- The nouser_xattr mount option is used on the cifs share.
- Gwenview accesses the files via smb:// URL instead of a cifs mount.

I don't know Gwenview's internals, but using its strace output as a guide, I
have written a potential reproducer. It succeeds at triggering slow getxattr()
calls, though not nearly as slow as those triggered by Gwenview. I can post it
if that would be helpful.