icmpv6: Fix mbuf change in mld

After r354748 mld_input() can change the mbuf. The new pointer
is never returned to icmp6_input() and when passed to
icmp6_rip6_input() the mbuf may no longer valid leading to
a panic.
Pass a pointer to the mbuf to mld_input() so we can return an
updated version in the non-error case.

Add a test sending an MLD packet case which will trigger this bug.

Pointyhat to: bz
Reported by: gallatin, thj
MFC After: 2 weeks
X-MFC with: r354748
Sponsored by: Netflix