devlink: share user_ptr pointer for both devlink and devlink_port

Ptr to devlink structure can be easily obtained from
devlink_port->devlink. So share user_ptr[0] pointer for both and leave
user_ptr[1] free for other users.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>