mlxsw: spectrum_router: Support IPv4 underlay decap

Unlike encapsulation, which is represented by a next hop forwarding to
an IPIP tunnel, decapsulation is a type of local route. It is created
for local routes whose prefix corresponds to the local address of one of
offloaded IPIP tunnels. When the tunnel is removed (i.e. all the encap
next hops are removed), the decap offload is migrated back to a trap for
resolution in slow path.

This patch assumes that decap route is already present when encap route
is added. A follow-up patch will fix this issue.

Note that this patch only supports IPv4 underlay. Support for IPv6
underlay will be subject to follow-up work apart from this patchset.

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>