Simplify GetLockNameFromTagType.

The old code is wrong, because it returns a pointer to an automatic
variable.  And it's also more clever than we really need to be
considering that the case it's worrying about should never happen.

src/backend/storage/lmgr/lmgr.c

@@ -1003,17 +1003,7 @@ DescribeLockTag(StringInfo buf, const LOCKTAG *tag)
 const char *
 GetLockNameFromTagType(uint16 locktag_type)
 {
-	const char *locktypename;
-	char		tnbuf[32];
-
-	if (locktag_type <= LOCKTAG_LAST_TYPE)
-		locktypename = LockTagTypeNames[locktag_type];
-	else
-	{
-		snprintf(tnbuf, sizeof(tnbuf), "unknown %d",
-				 (int) locktag_type);
-		locktypename = tnbuf;
-	}
-
-	return locktypename;
+	if (locktag_type > LOCKTAG_LAST_TYPE)
+		return "???";
+	return LockTagTypeNames[locktag_type];
 }