xnb: Don't pass SIOC{ADD,DEL}MULTI to ifmedia_ioctl().

ifmedia_ioctl() doesn't handle these requests, and this matches what
xn does.

Reviewed by:	royger
MFC after:	1 week
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D29296

sys/dev/xen/netback/netback.c

@@ -2296,6 +2296,7 @@ xnb_ioctl(struct ifnet *ifp, u_long cmd, caddr_t data)
 			break;
 		case SIOCADDMULTI:
 		case SIOCDELMULTI:
+			break;
 		case SIOCSIFMEDIA:
 		case SIOCGIFMEDIA:
 			error = ifmedia_ioctl(ifp, ifr, &xnb->sc_media, cmd);