CIFS: Fix lock consistensy bug in cifs_setlk

If we netogiate mandatory locking style, have a read lock and try
to set a write lock we end up with a write lock in vfs cache and
no lock in cifs lock cache - that's wrong. Fix it by returning
from cifs_setlk immediately if a error occurs during setting a lock.

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <smfrench@gmail.com>