syscall: dup the argument to fdopendir

syscall: dup the argument to fdopendir

fdopendir takes ownership of its file descriptor argument.
Getdirentries shouldn't do that, so dup the file descriptor
before passing to fdopendir.

Fixes #31269

Change-Id: Ie36be8fd6c59eb339dcc9f40228d4191fc1e5850
Reviewed-on: https://go-review.googlesource.com/c/go/+/170698
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>