pf: fix a crash on sendfile()

The network layer must not pass unmapped (M_EXTPG) mbufs to if_output()
of network interfaces without IFCAP_MEXTPG. pf should convert these
mbufs by mb_unmapped_to_ext() for such interfaces but it didn't.
The problem had occurred on sendfile because sendfile system call
uses unmapped mbufs for the file data.

Reported by: feld
Reviewed by: kp, glebius
Differential Revision: https://reviews.freebsd.org/D58021