block/rnbd-clt: Make path parameter optional for map_device

During map_device if the given session exists, then the path parameter is
not used. In such a case, the path parameter is redundant.

This commit makes the path parameter optional for map_device. When the
path parameter is not given, if the session exists then that is used to
establish the rtrs connection.

If the session does not exist, and the path parameter is also missing,
then map_device fails.

Signed-off-by: Md Haris Iqbal <haris.iqbal@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>