sched: restore discarded ifdef CONFIG_SCHED_WALT code

Code closed in ifdef CONFIG_SCHED_WALT blocks is not used in
msm-4.4 builds, hence in order to be as much as closer to
upstream and subsequently to have less merge conflicts in the
future, let's restore this code.

Restore below CONFIG_SCHED_WALT changes in file [1]:

  be832f69 sched: walt: Leverage existing
  ^^^^^^^ Discarded in dbad9b8f.

  efb86bd0 sched: Introduce Window Assisted Load Tracking (WALT)
  ^^^^^^^ Restore only the block, which is modified by be832f69.
          Discarded in efbe378b.

dbad9b8f Merge android-4.4@89074de (v4.4.94) into msm-4.4
efbe378b Merge branch 'v4.4-16.09-android-tmp' into lsk-v4.4-16.09-android

[1] kernel/sched/sched.h

Change-Id: Ifd7e230b3b47dde61abf2472f092ff78d80b7427
Signed-off-by: Blagovest Kolenichev <bkolenichev@codeaurora.org>