bcache: don't pass a stack address to blkdev_get_by_path

bcache: don't pass a stack address to blkdev_get_by_path

sb is just an on-stack pointer that can easily be reused by other calls.
Switch to use the bcache-wide bcache_kobj instead as there is no need to
claim per-bcache device anyway.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>