mlxsw: spectrum: router: Don't ignore IPMR notifications

Make the Spectrum router logic not ignore the RTNL_FAMILY_IPMR FIB
notifications.

Past commits added the IPMR VIF and MFC add/del notifications via the
fib_notifier chain. In addition, a code for handling these notifications in
the Spectrum router logic was added. Make the Spectrum router logic not
ignore these notifications and forward the requests to the Spectrum
multicast router offloading logic.

Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>