bridge: Fix inability to add non-vlan fdb entry

Bridge's default_pvid adds a vid by default, by which we cannot add a
non-vlan fdb entry by default, because br_fdb_add() adds fdb entries for
all vlans instead of a non-vlan one when any vlan is configured.

 # ip link add br0 type bridge
 # ip link set eth0 master br0
 # bridge fdb add 12:34:56:78:90:ab dev eth0 master temp
 # bridge fdb show brport eth0 | grep 12:34:56:78:90:ab
 12:34:56:78:90:ab dev eth0 vlan 1 static

We expect a non-vlan fdb entry as well as vlan 1:
 12:34:56:78:90:ab dev eth0 static

To fix this, we need to insert a non-vlan fdb entry if vlan is not
specified, even when any vlan is configured.

Fixes: 5be5a2df ("bridge: Add filtering support for default_pvid")
Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>