[LTP] lib: Use backoff polling to wait for loop device nodes

[Wake Liu]

Hi Li,

Thanks for the review.

I agree that LTP_TIMEOUT_MUL=3000 is an extreme case and unlikely to happen in practice.

However, since we already sent v7 which fixes this overflow cleanly (using 'unsigned long long' in the macro to prevent the wrap-around), and the AI reviewer has already passed it (Verdict - Reviewed), I think it's worth having the more robust version in v7.

Please take a look at v7.

Thanks,
Wake