vacuumlazy.c: Tweak local variable name.

Make a local variable name consistent with the name from its WAL record.

Extracted from a larger patch to deal with issues with how vacuumlazy.c
sets pages all-frozen.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WznuNGSzF8v6OsgjaC5aYsb3cZ6HW6MLm30X0d65cmSH6A@mail.gmail.com

src/backend/access/heap/vacuumlazy.c

@@ -2478,7 +2478,7 @@ lazy_vacuum_heap_page(LVRelState *vacrel, BlockNumber blkno, Buffer buffer,
 	VacDeadItems *dead_items = vacrel->dead_items;
 	Page		page = BufferGetPage(buffer);
 	OffsetNumber unused[MaxHeapTuplesPerPage];
-	int			uncnt = 0;
+	int			nunused = 0;
 	TransactionId visibility_cutoff_xid;
 	bool		all_frozen;
 	LVSavedErrInfo saved_err_info;
@@ -2508,10 +2508,10 @@ lazy_vacuum_heap_page(LVRelState *vacrel, BlockNumber blkno, Buffer buffer,
 
 		Assert(ItemIdIsDead(itemid) && !ItemIdHasStorage(itemid));
 		ItemIdSetUnused(itemid);
-		unused[uncnt++] = toff;
+		unused[nunused++] = toff;
 	}
 
-	Assert(uncnt > 0);
+	Assert(nunused > 0);
 
 	/* Attempt to truncate line pointer array now */
 	PageTruncateLinePointerArray(page);
@@ -2527,13 +2527,13 @@ lazy_vacuum_heap_page(LVRelState *vacrel, BlockNumber blkno, Buffer buffer,
 		xl_heap_vacuum xlrec;
 		XLogRecPtr	recptr;
 
-		xlrec.nunused = uncnt;
+		xlrec.nunused = nunused;
 
 		XLogBeginInsert();
 		XLogRegisterData((char *) &xlrec, SizeOfHeapVacuum);
 
 		XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);
-		XLogRegisterBufData(0, (char *) unused, uncnt * sizeof(OffsetNumber));
+		XLogRegisterBufData(0, (char *) unused, nunused * sizeof(OffsetNumber));
 
 		recptr = XLogInsert(RM_HEAP2_ID, XLOG_HEAP2_VACUUM);