CLEANUP: Replace repeated code to count usable servers with be_usable_srv()

2 places were using an open-coded implementation of this function to count
available servers. Note that the avg_queue_size() fetch didn't check that
the proxy was in STOPPED state so it would possibly return a wrong server
count here but that wouldn't impact the returned value.

Signed-off-by: Nenad Merdanovic <nmerdan@haproxy.com>
diff --git a/src/backend.c b/src/backend.c
index 5e2b8fc..b0e0332 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1614,14 +1614,7 @@
 	smp->data.type = SMP_T_SINT;
 	px = args->data.prx;
 
-	if (px->state == PR_STSTOPPED)
-		smp->data.u.sint = 0;
-	else if (px->srv_act)
-		smp->data.u.sint = px->srv_act;
-	else if (px->lbprm.fbck)
-		smp->data.u.sint = 1;
-	else
-		smp->data.u.sint = px->srv_bck;
+	smp->data.u.sint = be_usable_srv(px);
 
 	return 1;
 }
@@ -1780,12 +1773,7 @@
 	smp->data.type = SMP_T_SINT;
 	px = args->data.prx;
 
-	if (px->srv_act)
-		nbsrv = px->srv_act;
-	else if (px->lbprm.fbck)
-		nbsrv = 1;
-	else
-		nbsrv = px->srv_bck;
+	nbsrv = be_usable_srv(px);
 
 	if (nbsrv > 0)
 		smp->data.u.sint = (px->totpend + nbsrv - 1) / nbsrv;