jbd2: Update locking coments

(cherry picked from commit 86db97c8)

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: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>