Add u_int nstart, nend counters to consumer and providers so we will not

have to examine the stats structure to tell if we have outstanding I/O
requests.

Making them u_int improves the chance of atomic updates to them,
but risks roll-over.  Since the only interesting property is if
they are equal or not, this is not an issue.

sys/geom/geom.h

@@ -152,6 +152,7 @@ struct g_consumer {
 	struct g_event		*event;
 	int			spoiled;
 	struct g_stat		*stat;
+	u_int			nstart, nend;
 };
 
 /*
@@ -173,6 +174,7 @@ struct g_provider {
 	u_int			stripesize;
 	u_int			stripeoffset;
 	struct g_stat		*stat;
+	u_int			nstart, nend;
 	u_int			flags;
 #define G_PF_CANDELETE		0x1
 };