e1000: Fix lem/em UDP rx csum offload

Rebase on igb code and unify lem/em implementations.

PR: 257642
Reported by: Nick Reilly <nreilly@blackberry.com>
Reviewed by: karels, emaste
Tested by: Nick Reilly <nreilly@blackberry.com>
Approved by: grehan
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31449