Save pg_stat_statements statistics file into $PGDATA/pg_stat directory at shutdown.

187492b6c2e8cafc5b39063ca3b67846e8155d24 changed pgstat.c so that
the stats files were saved into $PGDATA/pg_stat directory when the server
was shutdowned. But it accidentally forgot to change the location of
pg_stat_statements permanent stats file. This commit fixes pg_stat_statements
so that its stats file is also saved into $PGDATA/pg_stat at shutdown.

Since this fix changes the file layout, we don't back-patch it to 9.3
where this oversight was introduced.

contrib/pg_stat_statements/pg_stat_statements.c

@@ -80,7 +80,7 @@
 PG_MODULE_MAGIC;
 
 /* Location of permanent stats file (valid when database is shut down) */
-#define PGSS_DUMP_FILE	"global/pg_stat_statements.stat"
+#define PGSS_DUMP_FILE	PGSTAT_STAT_PERMANENT_DIRECTORY "/pg_stat_statements.stat"
 
 /*
  * Location of external query text file.  We don't keep it in the core

src/backend/postmaster/pgstat.c

@@ -67,14 +67,6 @@
 #include "utils/tqual.h"
 
 
-/* ----------
- * Paths for the statistics files (relative to installation's $PGDATA).
- * ----------
- */
-#define PGSTAT_STAT_PERMANENT_DIRECTORY		"pg_stat"
-#define PGSTAT_STAT_PERMANENT_FILENAME		"pg_stat/global.stat"
-#define PGSTAT_STAT_PERMANENT_TMPFILE		"pg_stat/global.tmp"
-
 /* ----------
  * Timer definitions.
  * ----------

src/include/pgstat.h

@@ -20,6 +20,14 @@
 #include "utils/relcache.h"
 
 
+/* ----------
+ * Paths for the statistics files (relative to installation's $PGDATA).
+ * ----------
+ */
+#define PGSTAT_STAT_PERMANENT_DIRECTORY		"pg_stat"
+#define PGSTAT_STAT_PERMANENT_FILENAME		"pg_stat/global.stat"
+#define PGSTAT_STAT_PERMANENT_TMPFILE		"pg_stat/global.tmp"
+
 /* Default directory to store temporary statistics data in */
 #define PG_STAT_TMP_DIR		"pg_stat_tmp"