[LLC]: Use skb_reset_mac_header in llc_alloc_frame

skb->head is equal to skb->data after alloc_skb, so reset the mac header while
this is true, i.e. before skb_reserve.

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