scsi: target: Convert to bdev_open_by_path()

Convert iblock and pscsi drivers to use bdev_open_by_path() and pass the
handle around.

CC: target-devel@vger.kernel.org
CC: linux-scsi@vger.kernel.org
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/20230927093442.25915-15-jack@suse.cz
Reviewed-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>