mlxsw: spectrum_router: Make nexthops typed

In the router, some next hops may reference an encapsulating netdevice,
such as GRE or IPIP. To properly offload these next hops, mlxsw needs to
keep track of whether a given next hop is a regular Ethernet entry, or
an IP-in-IP tunneling entry.

To facilitate this book-keeping, add a type field to struct
mlxsw_sp_nexthop. There is, as of this patch, only one next hop type:
MLXSW_SP_NEXTHOP_TYPE_ETH. Follow-up patches will introduce the IP-in-IP
variant.

There are several places where next hops are initialized in the IPv4
path. Instead of replicating the logic at every one of them, factor it
out to a function mlxsw_sp_nexthop4_type_init(). The corresponding fini
is actually protocol-neutral, so put it to mlxsw_sp_nexthop_type_fini(),
but create a corresponding protocoled _fini function that dispatches to
the protocol-neutral one.

The IPv6 path is simpler, but for symmetry with IPv4, create the same
suite of functions with corresponding logic.

Signed-off-by: Petr Machata <petrm@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>