LinuxKPI: skbuff: remove assumption about mac_header

It seems the mac_header can be set to offset 0 for frames received.
Remove the warning splattering messages to the console for each packet.

Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)

(cherry picked from commit f0e59b6903c881859d7f4eaf269b2336b774c7bc)
(cherry picked from commit 46e5d2b0c6e01fd0789a6ef543c8b9f52852301b)