LinuxKPI: skbuff: handle dev_alloc_skb() correctly

dev_alloc_skb() comapred to alloc_skb() reserves some headroom
at the beginning of the skb which is used by drivers.
Split the code for the two cases and reserve NET_SKB_PAD space,
which should at least be 32 octets.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days