sched: init this_load to max in find_idlest_group

Init this_load to max value instead of 0 in find_idlest_group.
If the local group is skipped because it doesn't have allowed CPUs, this_load
stays to 0,  no idlest group will be returned and the selected CPU will be a
not allowed one (which will be replaced in select_fallback_rq by a random
one). With a default value set to max, we will use the idlest group even if we
skip the local_group.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: flar2 <asegaert@gmail.com>