libext2fs: relock CACHE_MTX after calling ->write_error

libext2fs: relock CACHE_MTX after calling ->write_error

In the UNIX I/O manager, we drop CACHE_MTX before calling the
->write_error handler in case it decides to retry the failed write.
Therefore, we must retake the lock after it returns, to ensure
consistent lock state when flush_cached_blocks returns.

Cc: <linux-ext4@vger.kernel.org> # v1.46.6
Fixes: 0e0c7537eb5fdc ("libext2fs: unix_io: fix_potential error path deadlock in flush_cached_blocks()")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>