mlxsw: spectrum_router: Make FIB node retrieval family agnostic

A FIB node is an entity which stores routes sharing the same prefix and
length. The data structure itself is already family agnostic, but we
make some of its operations agnostic as well and thus re-use them for
IPv6 offload.

Instead of passing an IPv4-specific structure to fib4_node_get(), pass
general routing parameters and rename the function accordingly.

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