scsi: don't add scsi_device if its already visible

If LLD has added scsi device (by calling scsi_add_device) before scheduling
async scsi_scan_host then scsi_finish_async_scan() will end up calling
scsi_sysfs_add_sdev for scsi device which was already added by LLD.
This patch fixes this issue by skipping the call to scsi_sysfs_add_sdev()
if it's already visible to rest of the kernel.

Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>