s390/qeth: fold VLAN handling into l3_rebuild_skb()

Move the overly complicated VLAN processing from the L3 RX handler into
its l3_rebuild_skb() helper. No change in functionality.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Reviewed-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>