8021q: fix mac_len recomputation in vlan_untag()

skb_reset_mac_len() relies on the value of the skb->network_header pointer,
therefore we must wait for such pointer to be recalculated before computing
the new mac_len value.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: David S. Miller <davem@davemloft.net>