bridge: Fix br_should_learn to check vlan_enabled

As Toshiaki Makita pointed out, the BRIDGE_INPUT_SKB_CB will
not be initialized in br_should_learn() as that function
is called only from br_handle_local_finish().  That is
an input handler for link-local ethernet traffic so it perfectly
correct to check br->vlan_enabled here.

Reported-by: Toshiaki <Makita&lt;toshiaki.makita1@gmail.com>
Fixes: 20adfa1a bridge: Check if vlan filtering is enabled only once.
Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>