sched/walt: consider average max_nr when compare max_nr

There is frequent Ping-Pong effect if only consider max_nr
in core control, so also consider average max_nr which in
many windows, this will make core control more smooth.

Change-Id: Ic5f3285db604357f7c953b02ec40379ac49b95e0
Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org>