[release-branch.go1.23] syscall: on exec failure, close pidfd

For golang#69284
Fixes golang#69402

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>
(cherry picked from commit 8926ca9)
Reviewed-on: https://go-review.googlesource.com/c/go/+/613616
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/syscall/exec_linux.go

@@ -839,3 +839,11 @@ func doCheckClonePidfd(pidfd *int32) (pid uintptr, errno Errno) {
 		RawSyscall(SYS_EXIT_GROUP, 0, 0, 0)
 	}
 }
+
+// forkAndExecFailureCleanup cleans up after an exec failure.
+func forkAndExecFailureCleanup(attr *ProcAttr, sys *SysProcAttr) {
+	if sys.PidFD != nil && *sys.PidFD != -1 {
+		Close(*sys.PidFD)
+		*sys.PidFD = -1
+	}
+}