net: ipmr: Add ipmr_rtm_getroute

Add to RTNL_FAMILY_IPMR, RTM_GETROUTE the ability
to retrieve one S,G mroute from a specified table.

*,G will return mroute information for just that
particular mroute if it exists.  This is because
it is entirely possible to have more S's then
can fit in one skb to return to the requesting
process.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>