IPv6: Fix a panic in the nd6 code with unmapped mbufs.

If the neighbor entry for an IPv6 TCP session using unmapped
mbufs times out, IPv6 will send an icmp6 dest. unreachable
message. In doing this, it will try to do a software checksum
on the reflected packet. If this is a TCP session using unmapped
mbufs, then there will be a kernel panic.

To fix this, just free packets with unmapped mbufs, rather
than sending the icmp.

Reviewed by: np, rrs
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D24821