mlxsw: spectrum_router: Refresh nexthop neighbour when it becomes dead

The driver tries to periodically refresh neighbours that are used to
reach nexthops. This is done by periodically calling neigh_event_send().

However, if the neighbour becomes dead, there is nothing we can do to
return it to a connected state and the above function call is basically
a NOP.

This results in the nexthop never being written to the device's
adjacency table and therefore never used to forward packets.

Fix this by dropping our reference from the dead neighbour and
associating the nexthop with a new neigbhour which we will try to
refresh.

Fixes: a7ff87ac ("mlxsw: spectrum_router: Implement next-hop routing")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reported-by: Alex Veber <alexve@mellanox.com>
Tested-by: Alex Veber <alexve@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>