Count buffers dirtied due to hints in pgBufferUsage.shared_blks_dirtied.

Previously, such buffers weren't counted, with the possible result that
EXPLAIN (BUFFERS) and pg_stat_statements would understate the true
number of blocks dirtied by an SQL statement.

Back-patch to 9.2, where this counter was introduced.

Amit Kapila

src/backend/storage/buffer/bufmgr.c

@@ -2474,6 +2474,7 @@ SetBufferCommitInfoNeedsSave(Buffer buffer)
 		{
 			/* Do vacuum cost accounting */
 			VacuumPageDirty++;
+			pgBufferUsage.shared_blks_dirtied++;
 			if (VacuumCostActive)
 				VacuumCostBalance += VacuumCostPageDirty;
 		}