CIFS: Implement caching mechanism for mandatory brlocks

If we have an oplock and negotiate mandatory locking style we handle
all brlock requests on the client.

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