sched: Initialize env->loop variable to 0

load_balance() function does not explicitly initialize env->loop
variable to 0. As a result, there is a vague possibility of
move_tasks() hitting a very long (unnecessary) loop when its unable to
move tasks from src_cpu. This can lead to unpleasant results like a
watchdog bark. Fix this by explicitly initializing env->loop variable
to 0 (in both load_balance() and active_load_balance_cpu_stop()).

Change-Id: I36b84c91a9753870fa16ef9c9339db7b706527be
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>