mlxsw: spectrum_router: Don't read 'nud_state' without lock

We periodically ask the neighbouring system to try and resolve
neighbours that are used for nexthops, but aren't currently resolved.

However, 'nud_state' is protected by the neighbour lock, so we shouldn't
access it without taking it. Instead, we can simply check the
'connected' field of the neighbour entry, which we update upon
NEIGH_UPDATE events.

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>