Ensure that IP fragments do not extend beyond IP_MAXPACKET.

Such fragments are obviously invalid, and when processed may end up
violating the sort order (by offset) of fragments of a given packet.
This doesn't appear to be exploitable, however.

Reviewed by: emaste
Discussed with: jtl
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17914