Fixed a bug ngx_http_upstream_fair_sched_score function.
Fixed a bug ngx_http_upstream_fair_sched_score function.

In SCHED_SCORE(nreq,delta) (((nreq) << SCHED_COUNTER_BITS) | (~(delta))) the ~(delta) would dominate the "bitwise or", making the calculated score ignore the number of requests the peer is currently processing.

The fix is to perform a bitwise mask on ~(delta) so it only affect the lower 20bits.
SCHED_SCORE(nreq,delta) (((nreq) << SCHED_COUNTER_BITS) | (~(delta) & SCHED_COUNTER_MAX))

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