Fix wal_writer_flush_after initializer value.

Commit a73952b7956 (new in 16) required default values in guc_table.c
and C variable initializers to match.  This one only matched when
XLOG_BLCKSZ == 8kB.  Fix by using the same expression in both places
with a new DEFAULT_XXX macro, as done for other GUCs.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGLNmLV=VrT==5MqnbARgx2ifRSFtdd8ofdfrdSLL3yv5A@mail.gmail.com

src/backend/postmaster/walwriter.c

@@ -68,7 +68,7 @@
  * GUC parameters
  */
 int			WalWriterDelay = 200;
-int			WalWriterFlushAfter = 128;
+int			WalWriterFlushAfter = DEFAULT_WAL_WRITER_FLUSH_AFTER;
 
 /*
  * Number of do-nothing loops before lengthening the delay time, and the

src/backend/utils/misc/guc_tables.c

@@ -2780,7 +2780,7 @@ struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_XBLOCKS
 		},
 		&WalWriterFlushAfter,
-		(1024 * 1024) / XLOG_BLCKSZ, 0, INT_MAX,
+		DEFAULT_WAL_WRITER_FLUSH_AFTER, 0, INT_MAX,
 		NULL, NULL, NULL
 	},
 

src/include/postmaster/walwriter.h

@@ -12,6 +12,8 @@
 #ifndef _WALWRITER_H
 #define _WALWRITER_H
 
+#define DEFAULT_WAL_WRITER_FLUSH_AFTER ((1024 * 1024) / XLOG_BLCKSZ)
+
 /* GUC options */
 extern PGDLLIMPORT int WalWriterDelay;
 extern PGDLLIMPORT int WalWriterFlushAfter;