fs: fix FileHandle::ClosePromise to return persisted Promise

Makes the FileHandle::ClosePromise() idempotent, always returning
the same Promise after it has already been successfully called
once. Avoids the possibility of accidental double close events.

Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/39331


Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>