block: refactor bd_start_claiming

Move the locking and assignment of bd_claiming from bd_start_claiming to
bd_prepare_to_claim.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>