syscall: fix closing of reordered FDs in plan9 ForkExec

After dup'ing file descriptors in syscall.ProcAttr.Files to pass
to the exec'ed process, the logic for closing the old descriptors
was incorrect and could close the new descriptor instead.

Fixes #57180

Change-Id: I7725f21a465ffba57050fe4e36f3d36ba181cfb2
Reviewed-on: https://go-review.googlesource.com/c/go/+/457115
Run-TryBot: David du Colombier <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: David du Colombier <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: millerresearch

src/syscall/exec_plan9.go

@@ -276,7 +276,7 @@ dirloop:
 
 	// Pass 3: close fd[i] if it was moved in the previous pass.
 	for i = 0; i < len(fd); i++ {
-		if fd[i] >= 0 && fd[i] != int(i) {
+		if fd[i] >= len(fd) {
 			RawSyscall(SYS_CLOSE, uintptr(fd[i]), 0, 0)
 		}
 	}