Fix fragmented UDP packets handling since rev.360967.

Consider IP_MF flag when checking length of the UDP packet to
match the declared value.

Sponsored by: Sippy Software, Inc.
Differential Revision: https://reviews.freebsd.org/D32363
MFC after: 2 weeks

(cherry picked from commit 461e6f23db3b9794e6af88b381b066a2c0463d1c)
(cherry picked from commit 73c5a2566dbb3ae57970b203d4de6fcf6088701c)

Approved by: so
Errata: EN-22:06.libalias