ipfw: fix checksum after NAT

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.

PR: 295057
Reviewed by: tuexen
MFC after: immediately
Differential Revision: https://reviews.freebsd.org/D57091