Count locked pages that don't need vacuuming as scanned.

Previously, if VACUUM skipped vacuuming a page because it's pinned, it
didn't count that page as scanned. However, that meant that relfrozenxid
was not bumped up either, which prevented anti-wraparound vacuum from
doing its job.

Report by Миша Тюрин, analysis and patch by Sergey Burladyn and Jeff Janes.
Backpatch to 9.2, where the skip-locked-pages behavior was introduced.

src/backend/commands/vacuumlazy.c

@@ -602,6 +602,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 			if (!lazy_check_needs_freeze(buf))
 			{
 				UnlockReleaseBuffer(buf);
+				vacrelstats->scanned_pages++;
 				continue;
 			}
 			LockBuffer(buf, BUFFER_LOCK_UNLOCK);