ipfilter: Obtain the interface name more efficiently

Rather than use a kmem read to determine the interface name used by a
nat_t structure through a pointer, nat_ipfs->netif->if_xname, obtain it
directly from nat_ifnames in the nat_t structure itself using the new
FORMAT_IF macro.

(cherry picked from commit ec793543feff027f0de9dff3708ab86f5e567d15)