[SK_BUFF]: Use skb_reset_network_header where the skb_pull return was being used

But only in the cases where its a newly allocated skb, i.e. one where skb->tail
is equal to skb->data, or just after skb_reserve, where this requirement is
maintained.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>