scsi: serialize ->rescan against ->remove

Lock the device embedded in the scsi_device to protect against
concurrent calls to ->remove.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>