LinuxKPI: add a work-in-progress skbuff implementation

This is a work-in-progress implementation of sk_buff compat code
used for wireless drivers only currently.
Bring in this version of the code as it has proven to be good enough
to have packets going for a few months.

The current implementation has several drawbacks including the need
for us to copy data between sk_buffs and mbufs.
Do not rely on the internals of this implementation. They are highly
likely to change as we will improve the integration to FreeBSD mbufs.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days