cobalt/posix/signal: Fix return code on siginfo copy errors

The put_siginfo functions return a non-zero value on error. We have to
convert that into proper -EFAULT before returning.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kernel/cobalt/posix/signal.c

@@ -376,7 +376,7 @@ out:
 		/* no more ref. to sigp beyond this point. */
 	}
 
-	return ret ?: sig;
+	return ret ? -EFAULT : sig;
 fail:
 	xnlock_put_irqrestore(&nklock, s);