rnbd-srv: Convert to use bdev_open_by_path()

Convert rnbd-srv to use bdev_open_by_path() and pass the handle
around.

CC: Jack Wang <jinpu.wang@ionos.com>
CC: "Md. Haris Iqbal" <haris.iqbal@ionos.com>
Acked-by: "Md. Haris Iqbal" <haris.iqbal@ionos.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230927093442.25915-6-jack@suse.cz
Signed-off-by: Christian Brauner <brauner@kernel.org>