mlxsw: spectrum_router: Make IPMR-related APIs family agnostic

spectrum_router and spectrum_mr have several APIs that are used to
manipulate configurations originating from ipmr fib notifications.
Following previous patches all the protocol-specifics that are necessary
for the configuration are hidden within spectrum_mr. This allows us to
clean the API and make sure that other than choosing the mr_table based
on the fib notification family, spectrum_router wouldn't care about the
source of the notification when passing it onward to spectrum_mr.

This would later allow us to leverage the same code for fib
notifications originating from ip6mr.

Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>