fs: simplify freeze_bdev/thaw_bdev

Store the frozen superblock in struct block_device to avoid the awkward
interface that can return a sb only used a cookie, an ERR_PTR or NULL.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Chao Yu <yuchao0@huawei.com>		[f2fs]
Signed-off-by: Jens Axboe <axboe@kernel.dk>