Fix panic in IPv6 multicast code.

Add check that ifp supports IPv6 multicasts in in6_getmulti.
This fixes panic when user application tries to join into multicast
group on an interface that doesn't support IPv6 multicasts, like
IFT_PFLOG interfaces.

PR: 257302
Reviewed by: melifaro
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31420