bcache: transfer the sb_page reference to register_{bdev,cache}

Avoid an extra reference count roundtrip by transferring the sb_page
ownership to the lower level register helpers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>