Ensure that the vnet is set when calling pru_sockaddr() and
pru_peeraddr().

This is already true when called via kern_getsockname() and
kern_getpeername(). This patch sets it also, when they arecalled
via soo_fill_kinfo(). This is necessary, since the corresponding
functions for SCTP require the vnet to be set. Without this,
if a process having an wildcard bound SCTP socket is
terminated and a core is written, the kernel panics.

Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D13652