LinuxKPI: skbuff: start implementing skb_copy()

Implement skb_copy() with omissions of fragments and possibly other fields
for now. Should we hit frags at any point a log message will let us know.
For the few cases we need this currently this is enough.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 349b042b90057c1e9dbb64dfdb894fe03ea20a1d)