Each FIB node holds a linked list of routes sharing the same prefix and
length. In the case of IPv4 it's ordered according to table ID, metric
and TOS and only the first route in the list is actually programmed to
the device.

In case a gatewayed route is added somewhere in the list, then after its
nexthop group will be refreshed and become valid (due to the resolution
of its gateway), it'll mistakenly overwrite the existing entry.

Example:
192.168.200.0/24 dev enp3s0np3 scope link metric 1000 offload
192.168.200.0/24 via 192.168.100.1 dev enp3s0np3 metric 1000 offload

Both routes are marked as offloaded despite the fact only the first one
should actually be present in the device's table.

When refreshing the nexthop group, don't write the route to the device's
table unless it's the first in its node.

Fixes: 9aecce1c

 ("mlxsw: spectrum_router: Correctly handle identical routes")
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>