mount-util: drop exponential buffer growing in name_to_handle_at_loop()

mount-util: drop exponential buffer growing in name_to_handle_at_loop()

So, it appears name_to_handle_at() always returns the right buffer size
on EOVERFLOW, when it's returned due to a too small buffer. Let's rely
on that exclusively for sizing the buffer, and let's drop the
exponential buffer growing.

The new logic is now: if we see EOVERFLOW and the returned size has
increased, resize our buffer and try again. But if it didn't increase,
then propagate the EOVERFLOW as it likely has other causes.