Add the optional keyword "vlan" to bridge fdb show so the user can request
filtering by a specific vlan id. Currently the filtering is implemented
only in user-space. The argument name has been chosen to match the
add/del one - "vlan".

Example:
$ bridge fdb show vlan 400
52:54:00:bf:57:16 dev eth2 vlan 400 master br0 permanent
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>