jbd: update locking coments

Update information about locking in JBD revoke code.

Reported-by: Lin Tan <tammy000@gmail.com&gt;.>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>