ext4: fix race when reusing xattr blocks

ext4: fix race when reusing xattr blocks

When ext4_xattr_block_set() decides to remove xattr block the following
race can happen:

CPU1                                    CPU2
ext4_xattr_block_set()                  ext4_xattr_release_block()
  new_bh = ext4_xattr_block_cache_find()

                                          lock_buffer(bh);
                                          ref = le32_to_cpu(BHDR(bh)->h_refcount);
                                          if (ref == 1) {
                                            ...
                                            mb_cache_entry_delete();
                                            unlock_buffer(bh);
                                            ext4_free_blocks();
                                              ...
                                              ext4_forget(..., bh, ...);
                                                jbd2_journal_revoke(..., bh);

  ext4_journal_get_write_access(..., new_bh, ...)
    do_get_write_access()
      jbd2_journal_cancel_revoke(..., new_bh);

Later the code in ext4_xattr_block_set() finds out the block got freed
and cancels reusal of the block but the revoke stays canceled and so in
case of block reuse and journal replay the filesystem can get corrupted.
If the race works out slightly differently, we can also hit assertions
in the jbd2 code.

Fix the problem by making sure that once matching mbcache entry is
found, code dropping the last xattr block reference (or trying to modify
xattr block in place) waits until the mbcache entry reference is
dropped. This way code trying to reuse xattr block is protected from
someone trying to drop the last reference to xattr block.

Reported-and-tested-by: Ritesh Harjani <ritesh.list@gmail.com>
CC: stable@vger.kernel.org
Fixes: 82939d7999df ("ext4: convert to mbcache2")
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220712105436.32204-5-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>