md: compare bd_disk instead of bd_contains

To check for partitions of the same disk bd_contains works as well, but
bd_disk is way more obvious.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>