sched: Fix possibility of "stuck" reserved flag

check_for_migration() could mark a thread for migration (in
rq->push_task) and invoke active_load_balance_cpu_stop(). However that
thread could get migrated to another cpu by the time
active_load_balance_cpu_stop() runs, which could fail to clear
reserved flag for a cpu and drop task_sruct reference when cpu has
only one task (stopper thread running
active_load_balance_cpu_stop()). This would cause a cpu to have
reserved bit stuck, which prevents it from being used effectively.

Fix this by having active_load_balance_cpu_stop() drop reserved bit
always.

Change-Id: I2464a46b4ddb52376a95518bcc95dd9768e891f9
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>