sd: don't grab a device references from driver methods

The device model already takes care of races between ->remove and
->shutdown vs its other methods, and we now take care about locking
them out for ->rescan as well.

This is a partial revert of commit 39b7f1 ("[SCSI] sd: Fix refcounting").

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>