Update information about locking in JBD revoke code.

Reported-by: Lin Tan <tammy000@gmail.com>.
Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>