bridge: Fix br_should_learn to check vlan_enabled

bridge: Fix br_should_learn to check vlan_enabled

[ Upstream commit c095f248e63ada504dd90c90baae673ae10ee3fe ]

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<toshiaki.makita1@gmail.com>
Fixes: 20adfa1 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>