MFC r315516

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>

git-svn-id: svn://svn.freebsd.org/base/stable/10@316325 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f