Modify in6_pcbpurgeif0() to accept a pcbinfo structure rather than a pcb

Modify in6_pcbpurgeif0() to accept a pcbinfo structure rather than a pcb
list head structure; this improves congruence to IPv4, and also allows
in6_pcbpurgeif0() to lock the pcbinfo.  Modify in6_pcbpurgeif0() to lock
the pcbinfo before iterating the pcb list, use queue(9)'s LIST_FOREACH()
for the iteration, and to lock individual inpcb's while manipulating
them.

MFC after:	3 months