Swap destination and source arguments of two bcopy() calls.

Before committing the initial tcp_hostcache I changed them from memcpy()
to conform with FreeBSD style without realizing the difference in argument
definition.

This fixes hostcache operation for IPv6 (in general and explicitly IPv6
path mtu discovery) and T/TCP (RFC1644).

Submitted by: Taku YAMAMOTO <taku@cent.saitama-u.ac.jp>
Approved by: re (rwatson)