MFC r274017, r274088 and r275205:

Provide an on-stack temporary buffer for small IOCTL requests.
Avoiding a memory allocation per IOCTL request can give a significant
speedup for applications which heavily rely on IOCTLs.
MFC r274744.

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