MFC r315516

Change several constants used by the PIE algorithm from unsigned to signed.

• PIE_MAX_PROB is compared to variable of int64_t and the type promotion rules can cause the value of that variable to be treated as unsigned. If the value is actually negative, then the result of the comparsion is incorrect, causing the algorithm to perform poorly in some situations. Changing the constant to be signed cause the comparision to work correctly.
• PIE_SCALE is also compared to signed values. Fortunately they are also compared to zero and negative values are discarded so this is more of a cosmetic fix.
• PIE_DQ_THRESHOLD is only compared to unsigned values, but it is small enough that the automatic promotion to unsigned is harmless.

Submitted by: Rasool Al-Saadi <ralsaadi@swin.edu.au>