Change xl_hash_vacuum_one_page.ntuples from int to uint16.

This will create two bytes of padding space in xl_hash_vacuum_one_page which
can be used for future patches. This makes the datatype of
xl_hash_vacuum_one_page.ntuples same as gistxlogDelete.ntodelete which is
advisable as both are used for the same purpose.

Author: Bertrand Drouvot
Reviewed-by: Nathan Bossart
Discussion: https://postgr.es/m/b0e20c40-cb7a-fc1c-c607-2a78dac5021e@gmail.com

src/include/access/hash_xlog.h

@@ -251,13 +251,13 @@ typedef struct xl_hash_init_bitmap_page
 typedef struct xl_hash_vacuum_one_page
 {
 	TransactionId snapshotConflictHorizon;
-	int			ntuples;
+	uint16			ntuples;
 
 	/* TARGET OFFSET NUMBERS FOLLOW AT THE END */
 } xl_hash_vacuum_one_page;
 
 #define SizeOfHashVacuumOnePage \
-	(offsetof(xl_hash_vacuum_one_page, ntuples) + sizeof(int))
+	(offsetof(xl_hash_vacuum_one_page, ntuples) + sizeof(uint16))
 
 extern void hash_redo(XLogReaderState *record);
 extern void hash_desc(StringInfo buf, XLogReaderState *record);

src/include/access/xlog_internal.h

@@ -31,7 +31,7 @@
 /*
  * Each page of XLOG file has a header like this:
  */
-#define XLOG_PAGE_MAGIC 0xD111	/* can be used as WAL version indicator */
+#define XLOG_PAGE_MAGIC 0xD112	/* can be used as WAL version indicator */
 
 typedef struct XLogPageHeaderData
 {