[RFC PATCH 0/4 v2] Btrfs: Add readonly support to replace BUG_ON phrase

[liubo <liubo2009@cn.fujitsu.com>]

Btrfs has a number of BUG_ON()s, which may lead btrfs to unpleasant panic.
Meanwhile, they are very ugly and should be handled more propriately.

There are mainly two ways to deal with these BUG_ON()s.

1. For those errors which can be handled well by callers, we just return their
error number to callers.

2. For others, We can force the filesystem readonly when it hits errors, which
 is what this patchset has done. Replaced BUG_ON() with the interface provided
 in this patchset, we will get error infomation via dmesg. Since btrfs is now 
readonly, we can save our data safely and umount it, then a btrfsck is 
recommended.

By these ways, we can protect our filesystem from panic caused by those 
BUG_ONs.

We still need a incompat flag to make old kernels happy.

v1->v2:
- in order to avoid deadlock thing, move write super stuff from error handle
  path to umount time.
- remove BTRFS_SUPER_FLAG_VALID, just use BTRFS_SUPER_FLAG_ERROR to make it
  simple.
- add MS_RDONLY check at start of a transaction instead of commit transaction.

---
 fs/btrfs/ctree.h       |   19 ++++++++++
 fs/btrfs/disk-io.c     |   47 +++++++++++++++++++++++++-
 fs/btrfs/super.c       |   88 ++++++++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/transaction.c |    3 ++
 4 files changed, 156 insertions(+), 1 deletions(-)