jbd2: fix race when writing superblock

jbd2: fix race when writing superblock

The jbd2 superblock is lockless now, so there is probably a race
condition between writing it so disk and modifing contents of it, which
may lead to checksum error. The following race is the one case that we
have captured.

jbd2                                fsstress
jbd2_journal_commit_transaction
 jbd2_journal_update_sb_log_tail
  jbd2_write_superblock
   jbd2_superblock_csum_set         jbd2_journal_revoke
                                     jbd2_journal_set_features(revork)
                                     modify superblock
   submit_bh(checksum incorrect)

Fix this by locking the buffer head before modifing it.  We always
write the jbd2 superblock after we modify it, so this just means
calling the lock_buffer() a little earlier.

This checksum corruption problem can be reproduced by xfstests
generic/475.

Reported-by: zhangyi (F) <yi.zhang@huawei.com>
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>