Add an helper function to read the next valid VLAN entry for a given
port. It is used in the VID to FID conversion function to retrieve the
forwarding database assigned to a given VLAN port.

Finally update the FDB getnext operation to iterate on the next valid
port VLAN when the end of the current database is reached.
Signed-off-by: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>