Fix panic in network stack due to memory use after free in relation to
fragmented packets.

When sending IPv4 and IPv6 fragmented packets and a fragment is lost,
the mbuf making up the fragment will remain in the temporary hashed
fragment list for a while. If the network interface departs before the
so-called slow timeout clears the packet, the fragment causes a panic
when the timeout kicks in due to accessing a freed network interface
structure.

Make sure that when a network device is departing, all hashed IPv4 and
IPv6 fragments belonging to it, get freed.

Backtrace:
panic()
icmp6_reflect()

hlim = ND_IFINFO(m->m_pkthdr.rcvif)->chlim;
^^^^ rcvif->if_afdata[AF_INET6] is NULL.

icmp6_error()
frag6_freef()
frag6_slowtimo()
pfslowtimo()
softclock_call_cc()
softclock()
ithread_loop()

Differential Revision: https://reviews.freebsd.org/D19622
Reviewed by: bz (network), adrian
MFC after: 1 week
Sponsored by: Mellanox Technologies