bridge: link: allow filtering on bridge name

bridge: link: allow filtering on bridge name

When using 'brige link show' we can either dump all links enslaved to any bridge
(called without arg ) or display a single link (called with dev arg).
However there is no way to dummp all links of a single bridge.

To do so, this adds new optional 'master XXX' arg to 'bridge link show' command.
usage: bridge link show master br0

Signed-off-by: Nicolas Escande <nico.escande@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>