um: Fix return value of strnlen_user()

In case of an error it must not return -EFAULT.
Return 0 like all other archs do.

Reported-by: toralf.foerster@gmx.de
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/skas/uaccess.c

@@ -254,6 +254,6 @@ int strnlen_user(const void __user *str, int len)
 	n = buffer_op((unsigned long) str, len, 0, strnlen_chunk, &count);
 	if (n == 0)
 		return count + 1;
-	return -EFAULT;
+	return 0;
 }
 EXPORT_SYMBOL(strnlen_user);