When checksum offloading is used, IPFW needs to fix the checksum after libalias has done NAT. The ipfw_nat() function does so, but only for mbufs without a receiving interface. However, if, for example, the packet was sent inside a jail that used checksum offloading over an epair, ipfw still needs to fix the checksum even though the mbuf has set a receiving interface (epair).
This patch just removes the check whether a receiving interface is set.
It fixes a bug reported here: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=295057