fs: use consistent buffer array validation

This commit updates fs.writev() and fs.writevSync() to use the
same validation method as filehandle.writev().

PR-URL: https://github.com/nodejs/node/pull/29186


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>