Update information about locking in JBD2 revoke code. Inconsistency in
comments found by Lin Tan <tammy000@gmail.com>.

CC: Lin Tan <tammy000@gmail.com>.
Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>