sched: fix incorrect wait time and wait count statistics

Scheduler at present resets task's wait start timestamp when task migrates
to another rq.  This misleads scheduler itself into reporting less wait
time than actual by omitting time spent for waiting prior to migration and
also more wait count than actual by counting migration as wait end event
which can be seen by trace or /proc/<pid>/sched with CONFIG_SCHEDSTATS=y.

Carry forward migrating task's wait time prior to migration and don't count
migration as a wait-end event to fix such statistics error.

Change-Id: I0f6badf8072fc37826e4476ac2d1195e82b65bf1
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>