Set the current vnet inside the ioctl handler for ipfilter.

Without this fix, the vnet was NULL and would crash.
This fix is similar to what was done inside the ioctl handler for PF.

Tested by:
   (1) Boot a kernel with "options VIMAGE" enabled
   (2) Type:

   echo "map lo0 from 10.0.0.0/24 to ! 10.0.0.0/24 -> 127.0.0.1/32" > /etc/ipnat.rules ; service ipnat onerestart

PR: 176992
Differential Revision: https://reviews.freebsd.org/D1191
Reviewed by: cy