sched: reset blocked load decay_count during synchronization

sched: reset blocked load decay_count during synchronization

If an entity happens to sleep for less than one tick duration
the tracked load associated with that entity can be decayed by an
unexpectedly large amount if it is later migrated to a different
CPU. This can interfere with correct scheduling when entity load
is used for decision making.

The reason for this is that when an entity is dequeued and enqueued
quickly, such that se.avg.decay_count and cfs_rq.decay_counter
do not differ when that entity is enqueued again,
__synchronize_entity_decay skips the calculation step and also skips
clearing the decay_count. At a later time that entity may be
migrated and its load will be decayed incorrectly.

All users of this function expect decay_count to be zero'ed after
use.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Jon Medhurst <tixy@linaro.org>