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)