ether_demux: Defer stripping the Ethernet header.

This avoids having to undo it before invoking NetGraph's orphan input
hook.

Reviewed by: ae, melifaro
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D37510

(cherry picked from commit 79b679942352a7f10d759fdee1892aa251e0f439)