Rewrite NR_MAC UE_iterator() to limit variable scope

Especially in pf_dl()/pf_ul(), we use a first loop using
UE_iterator(), then iterate over a subset of UEs. It has happened a
couple of times that I was using variable "UE" (from the iterator), when
it should only be used in the first UE_iterator() loop.

Rewrite UE_iterator() to effectively limit the scope of the iterator
variables to the body of the UE_iterator. This should also make it
possible to reuse multiple UE_iterator()s with the same variables in
sequence.

openair2/LAYER2/NR_MAC_gNB/nr_mac_gNB.h

@@ -819,7 +819,7 @@ typedef struct {
   nr_beam_mode_t beam_mode;
 } NR_beam_info_t;
 
-#define UE_iterator(BaSe, VaR) NR_UE_info_t ** VaR##pptr=BaSe, *VaR; while ((VaR=*(VaR##pptr++)))
+#define UE_iterator(BaSe, VaR) for (NR_UE_info_t **VaR##pptr=BaSe, *VaR=*VaR##pptr; VaR; VaR=*(++VaR##pptr))
 
 typedef struct {
   /// current frame for DCI