LinuxKPI: skbuff: switch to using uma for the skb

Split allocating the skb header and its payload data memory into skb
header from uma and the data to whatever we used to do for it.

This is a first step which should allow us to one day at least in the
RX path take the data, attach it to an mbuf and send it up the stack
and just free the skbuff header.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days