zebra: When installing a mroute, allow it to flow

zebra: When installing a mroute, allow it to flow

Currently the mroute code was not allowing the mroute
to be sent to the dataplane.  This leaves us with a
situation where the routes being installed where never
being set as installed and additionally nht against
the mrib would not work if the route came into existence
after the nexthop tracking was asked for.

Turns out all the pieces where there to let this work.
Modify the code to pass it to the dplane and to send
it back up as having worked.

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