get-write-flag: don't test invariants

If we're not on windows, or don't have fmap support,
then just make this be a function that always returns 'w'.

If it is windows, and has fmap support, only test the size.

lib/get-write-flag.js

@@ -13,4 +13,5 @@ const { O_CREAT, O_TRUNC, O_WRONLY, UV_FS_O_FILEMAP = 0 } = fs.constants
 const fMapEnabled = isWindows && !!UV_FS_O_FILEMAP
 const fMapLimit = 512 * 1024
 const fMapFlag = UV_FS_O_FILEMAP | O_TRUNC | O_CREAT | O_WRONLY
-module.exports = size => (fMapEnabled && size < fMapLimit) ? fMapFlag : 'w'
+module.exports = !fMapEnabled ? () => 'w'
+  : size => size < fMapLimit ? fMapFlag : 'w'