fs: point isFd to isUint32

This commit updates the isFd function to call isUint32 instead of
doing the same thing.

PR-URL: nodejs#20330
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/fs.js

@@ -153,9 +153,7 @@ function makeStatsCallback(cb) {
   };
 }
 
-function isFd(path) {
-  return (path >>> 0) === path;
-}
+const isFd = isUint32;
 
 fs.Stats = Stats;