pimd: fix static mroute to also take into account the input interface

pimd: fix static mroute to also take into account the input interface

Allow the same group/source route to be configured on more than one interface.
Currently FRR doesn't allow adding the same mroute on different input interfaces.

Current behavior, if we have the following config:

```
interface eth1
   ip mroute eth0 239.0.0.1

interface eth2
   ip mroute eth0 239.0.0.1
```

Only one multicast route will be installed with an input interface of the last
interface configured.

Signed-off-by: Nathan Bahr <nbahr@atcorp.com>
Signed-off-by: Jafar Al-Gharaibeh <jafar@atcorp.com>