[LLC]: Use skb_reset_mac_header in llc_mac_hdr_init

skb_push updates and returns skb->data, so we can just call
skb_reset_mac_header after the call to skb_push.

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