ip: mroute: Add table output to show command

ip: mroute: Add table output to show command

When the user specifies `table all` or `table 0` to
the `ip mroute show` command we dump the entirety of
the known mroute tables.  Without some sort of
divisor to tell us what table we are looking at
the command is useless.

Add `Table: <vrf name>` to the output of 'ip mroute show table 0'

Follow the convention established by 'ip route show table 0'
for when to display

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>