An FDB entry with vlan_id 0 doesn't mean it is used in vlan 0, but used when
vlan_filtering is disabled.

There is inconsistency around NDA_VLAN whose payload is 0 - even if we add
an entry by RTM_NEWNEIGH without any NDA_VLAN, and even though adding an
entry with NDA_VLAN 0 is prohibited, we get an entry with NDA_VLAN 0 by
RTM_GETNEIGH.

Dumping an FDB entry with vlan_id 0 shouldn't include NDA_VLAN.
Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>