Track block level checksum failures in pg_stat_database

This adds a column that counts how many checksum failures have occurred
on files belonging to a specific database. Both checksum failures
during normal backend processing and those created when a base backup
detects a checksum failure are counted.

Author: Magnus Hagander
Reviewed by: Julien Rouhaud

src/backend/storage/page/bufpage.c

@@ -17,6 +17,7 @@
 #include "access/htup_details.h"
 #include "access/itup.h"
 #include "access/xlog.h"
+#include "pgstat.h"
 #include "storage/checksum.h"
 #include "utils/memdebug.h"
 #include "utils/memutils.h"
@@ -151,6 +152,8 @@ PageIsVerified(Page page, BlockNumber blkno)
 				 errmsg("page verification failed, calculated checksum %u but expected %u",
 						checksum, p->pd_checksum)));
 
+		pgstat_report_checksum_failure();
+
 		if (header_sane && ignore_checksum_failure)
 			return true;
 	}