cam: fix integer overflow during inquiry

From my understanding this could happen with iSCSI LUNs with
unusually long names.  The bug would make CAM fail to retrieve
the full inquiry data.  Instead of bumping the size of the local
variable, just use a macro.

Reviewed By:	imp, mav
Sponsored by:	NetApp, Inc.
Sponsored by:	Klara, Inc.
X-NetApp-PR:	#50
Differential Revision:	https://reviews.freebsd.org/D29991

sys/cam/scsi/scsi_xpt.c

@@ -1210,8 +1210,6 @@ out:
 
 			if (periph_qual == SID_QUAL_LU_CONNECTED ||
 			    periph_qual == SID_QUAL_LU_OFFLINE) {
-				u_int8_t len;
-
 				/*
 				 * We conservatively request only
 				 * SHORT_INQUIRY_LEN bytes of inquiry
@@ -1222,11 +1220,9 @@ out:
 				 * the amount of information the device
 				 * is willing to give.
 				 */
-				len = inq_buf->additional_length
-				    + offsetof(struct scsi_inquiry_data,
-                                               additional_length) + 1;
 				if (softc->action == PROBE_INQUIRY
-				    && len > SHORT_INQUIRY_LENGTH) {
+				    && SID_ADDITIONAL_LENGTH(inq_buf)
+				    > SHORT_INQUIRY_LENGTH) {
 					PROBE_SET_ACTION(softc, PROBE_FULL_INQUIRY);
 					xpt_release_ccb(done_ccb);
 					xpt_schedule(periph, priority);