pf: Handle unmapped mbufs when computing checksums

Approved by: re (cperciva)
PR: 254419
Reviewed by: gallatin, kp
Tested by: Igor A. Valkov <viaprog@gmail.com>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29378

(cherry picked from commit b93a796b06ec013a75a08ac43d8acf6aa94aa970)
(cherry picked from commit 5fcab6fbcf8b99d1420e681731a07670c38defe3)