ANDROID: sched/fair: initialise util_est values to 0 on fork

Since "sched/fair: Align PELT windows between cfs_rq and its se" the
upstream kernel has initialised the whole content of sched_avg to zero
on fork. When util_est was backported, we missed this and so ended up
with util_est values copied from the parent task.

Add the zero initialisation which is present upstream and ensure that
util_est values always start from a known point.

Fixes: 700f1172 ("BACKPORT: sched/fair: Add util_est on top of PELT")
Reported-by: Puja Gupta <pujag@quicinc.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
Cc: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Todd Kjos <tkjos@google.com>
Cc: Saravana Kannan <skannan@codeaurora.org>
Change-Id: I06995e4320d606a52761d0e773baf28fcd1e2680
Signed-off-by: Chris Redpath <chris.redpath@arm.com>