scsi_dh: return SCSI_DH_NOTCONN in scsi_dh_activate()

When calling scsi_dh_activate() we should be returning
SCSI_DH_NOTCONN if the device handler couldn't be attached.

Reviewed-by: Bart van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>