netinet6: perform out-of-bounds check for lo0 multicast statitics

[melifaro]

Summary

Currently, some per-mbuf multicast statistic is stored in 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.