netinet6: perform out-of-bounds check for loX multicast statistics

Currently, some per-mbuf multicast statistics is stored in
the per-interface ip6stat.ip6s_m2m[] array of size 32 (IP6S_M2MMAX).
Check that loopback ifindex falls within 0.. IP6S_M2MMAX-1 range to
avoid silent data corruption. The latter cat happen with large
number of VNETs.

Reviewed by: glebius
Differential Revision: https://reviews.freebsd.org/D35715
MFC after: 2 weeks