cam: Enforce real priorities in xpt_action for queued ccbs.

All queued CCBs should be created with a real priority (one that's not
CAM_PRIORITY_NONE). Recently, I introduced a bug that revealed a latent
MMC bug where it would stop enumerating due to a bad priority. Add an
assert to catch that (the other bug in mmc_da that it found has been
fixed).

Sponsored by:		Netflix

sys/cam/cam_xpt.c

@@ -2515,6 +2515,15 @@ xpt_action(union ccb *start_ccb)
 	    ("xpt_action: func %#x %s\n", start_ccb->ccb_h.func_code,
 		xpt_action_name(start_ccb->ccb_h.func_code)));
 
+	/*
+	 * Either it isn't queued, or it has a real priority. There still too
+	 * many places that reuse CCBs with a real priority to do immediate
+	 * queries to do the other side of this assert.
+	 */
+	KASSERT((start_ccb->ccb_h.func_code & XPT_FC_QUEUED) == 0 ||
+	    start_ccb->ccb_h.pinfo.priority != CAM_PRIORITY_NONE,
+	    ("%s: queued ccb and CAM_PRIORITY_NONE illegal.", __func__));
+
 	start_ccb->ccb_h.status = CAM_REQ_INPROG;
 	(*(start_ccb->ccb_h.path->bus->xport->ops->action))(start_ccb);
 }