zebra: Send correct default vrf tableid for MROUTE stats

zebra: Send correct default vrf tableid for MROUTE stats

So the linux kernel uses the RT_TABLE_MAIN for the table
id used for ip routing.  The multicast routing tables use
RT_TABLE_DEFAULT.  We changed the internal code of zebra_vrf
a few months back to use RT_TABLE_MAIN as the tableid to
use.  This caused the pim sg stats to stop working because
of the kernel bug where it uses a different table
for ip routing and ip multicast.

Put a bit of a special case in to do the right thing.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>