ipfilter: Fix struct ifnet pointer type

The fr_info struct contains a summary of a packet. One of its fields
is a pointer to the ifnet struct the packet arrived on. It is pointed
to by a void* because ipfilter supports multiple O/Ses. Unfortunately
this makes it difficult it examine with DTrace. Defining fin_ifp as a
pointer to an ifnet struct makes the struct it points to using a DTrace
script possible.

MFC after: 1 week