nvmet: Convert to bdev_open_by_path()

Convert nvmet to use bdev_open_by_path() and pass the handle around.

CC: linux-nvme@lists.infradead.org
Acked-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230927093442.25915-13-jack@suse.cz
Signed-off-by: Christian Brauner <brauner@kernel.org>