syscall: on exec failure, close pidfd

syscall: on exec failure, close pidfd

Fixes #69284

Change-Id: I6350209302778ba5e44fa03d0b9e680d2b4ec192
Reviewed-on: https://go-review.googlesource.com/c/go/+/611495
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: roger peppe <rogpeppe@gmail.com>
Reviewed-by: Tim King <taking@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>