ip_input: packet filters shall not modify m_pkthdr.rcvif

Quick review confirms that they do not, also IPv6 doesn't expect
such a change in mbuf. In IPv4 this appeared in 0aade26e6d061,
which doesn't seem to have a valid explanation why.

Reviewed by: donner, kp, melifaro
Differential revision: https://reviews.freebsd.org/D32913