Fix new warnings from GCC 7

This addresses the new warning types -Wformat-truncation
-Wformat-overflow that are part of -Wall, via -Wformat, in GCC 7.

src/backend/utils/cache/relcache.c

@@ -5160,7 +5160,7 @@ RelationCacheInitFileRemove(void)
 	const char *tblspcdir = "pg_tblspc";
 	DIR		   *dir;
 	struct dirent *de;
-	char		path[MAXPGPATH];
+	char		path[MAXPGPATH + 10 + sizeof(TABLESPACE_VERSION_DIRECTORY)];
 
 	/*
 	 * We zap the shared cache file too.  In theory it can't get out of sync
@@ -5202,7 +5202,7 @@ RelationCacheInitFileRemoveInDir(const char *tblspcpath)
 {
 	DIR		   *dir;
 	struct dirent *de;
-	char		initfilename[MAXPGPATH];
+	char		initfilename[MAXPGPATH * 2];
 
 	/* Scan the tablespace directory to find per-database directories */
 	dir = AllocateDir(tblspcpath);