Track SLRU page hits in SimpleLruReadPage_ReadOnly

SLRU page hits were tracked only in SimpleLruReadPage, but that's not
enough because we may hit the page in SimpleLruReadPage_ReadOnly in
which case we don't call SimpleLruReadPage at all.

Reported-by: Kuntal Ghosh
Discussion: https://postgr.es/m/20200119143707.gyinppnigokesjok@development

src/backend/access/transam/slru.c

@@ -491,6 +491,10 @@ SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno, TransactionId xid)
 		{
 			/* See comments for SlruRecentlyUsed macro */
 			SlruRecentlyUsed(shared, slotno);
+
+			/* update the stats counter of pages found in the SLRU */
+			pgstat_count_slru_page_hit(ctl);
+
 			return slotno;
 		}
 	}