nfs/blocklayout: Convert to use bdev_open_by_dev/path()

nfs/blocklayout: Convert to use bdev_open_by_dev/path()

Convert block device handling to use bdev_open_by_dev/path() and pass
the handle around.

CC: linux-nfs@vger.kernel.org
CC: Trond Myklebust <trond.myklebust@hammerspace.com>
CC: Anna Schumaker <anna@kernel.org>
Acked-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230927093442.25915-25-jack@suse.cz
Signed-off-by: Christian Brauner <brauner@kernel.org>