contrib/fs-idmap: Reap childs

This is what we should do, although in practice this probably won't be a
big issue as the parent also exits.

While we are there, instead of waiting for the child to finish, kill it
if we did everything we wanted to do.

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

contrib/cmd/fs-idmap/fs-idmap.go

@@ -31,6 +31,10 @@ func main() {
 	if err := cmd.Start(); err != nil {
 		log.Fatalf("failed to run the helper binary: %v", err)
 	}
+	defer func() {
+		_ = cmd.Process.Kill()
+		_ = cmd.Wait()
+	}()
 
 	path := fmt.Sprintf("/proc/%d/ns/user", cmd.Process.Pid)
 	var userNsFile *os.File