Add the optional keyword "vid" to bridge mdb 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 - "vid".

Example:
$ bridge mdb show vid 200
dev br0 port eth2 grp 239.0.0.1 permanent vid 200
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>