sched: Avoid needless migration

Restrict check_for_migration() to operate on fair_sched class tasks
only.

Also check_for_migration() can result in a call to select_best_cpu()
to look for a better cpu for currently running task on a cpu. However
select_best_cpu() can end up suggesting a cpu that is not necessarily
better than the cpu on which task is running currently. This will
result in unnecessary migration. Prevent that from happening.

Change-Id: I391cdda0d7285671d5f79aa2da12eaaa6cae42d7
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>