ext4: annotate data race in start_this_handle()

Access to journal->j_running_transaction is not protected by appropriate
lock and thus is racy. We are well aware of that and the code handles
the race properly. Just add a comment and data_race() annotation.

Cc: stable@kernel.org
Reported-by:  <syzbot+30774a6acf6a2cf6d535@syzkaller.appspotmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210406161804.20150-1-jack@suse.cz


Signed-off-by: Theodore Ts'o <tytso@mit.edu>