mlxsw: spectrum: Support decap-only IP-in-IP tunnels

Current code for offloading IP-in-IP tunneling assumes that there is no
decap without encap. But that's never true for IPv6 overlays, and is not
true for IPv4 ones either, if net.ipv4.conf.*.rp_filter is unset.

To support decap-only tunnels, an IPIP entry is now created as soon as
an offloadable tunneling device is created. When that netdevice is up'd,
a decap route is looked up and possibly offloaded. Thus decap is not
handled implicitly as part of mlxsw_sp_ipip_entry_get() call anymore,
but needs to be done explicitly after the get, if desired.

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>