sched/fair: Mask UTIL_AVG_UNCHANGED usages

The _task_util_est() is mainly used to add/remove the task contribution
to/from the rq's estimated utilization at task enqueue/dequeue time.
In both cases we ensure the UTIL_AVG_UNCHANGED flag is set to keep
consistency between enqueue and dequeue time while still being
transparent to update_load_avg calls which will eventually reset the
flag.

Let's move the flag forcing within _task_util_est() itself so that we
can simplify calling code by hiding that estimated utilization
implementation detail into one of its internal functions.

This will affect also the "public" API task_util_est() but we know that
the flag will (eventually) impact just on the LSB of the estimated
utilization, thus it's certainly acceptable.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Perret <quentin.perret@arm.com>
Cc: Steve Muckle <smuckle@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Todd Kjos <tkjos@google.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Link: http://lkml.kernel.org/r/20181105145400.935-3-patrick.bellasi@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>