igb: only use vlan_gro_receive if vlans are registered

igb: only use vlan_gro_receive if vlans are registered

This change makes it so that vlan_gro_receive is only used if vlans have been
registered to the adapter structure.  Previously we were just sending all vlan
tagged frames in via this function but this results in a null pointer
dereference when vlans are not registered.

[ This fixes bugzilla entry 15582 -Eric Dumazet]

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>