Fix "camcontrol rescan" with SATA drives behind a SAS controller

A bug in CAM's serial number hash logic resulted in SATA drives behind a SAS
controller getting removed and readded anytime the drive was rescanned for
any reason.

PR:		212914
Submitted by:	kadesai
Reported by:	kadesai
Reviewed by:	asomers, ken
MFC after:	4 weeks
Sponsored by:	Spectra Logic Corp

sys/cam/scsi/scsi_xpt.c

@@ -1626,8 +1626,8 @@ probedone(struct cam_periph *periph, union ccb *done_ccb)
 				  sizeof(struct scsi_inquiry_data));
 
 			if (have_serialnum)
-				MD5Update(&context, serial_buf->serial_num,
-					  serial_buf->length);
+				MD5Update(&context, path->device->serial_num,
+					  path->device->serial_num_len);
 
 			MD5Final(digest, &context);
 			if (bcmp(softc->digest, digest, 16) == 0)