sched: avoid pushing tasks to an offline CPU

Currently active_load_balance_cpu_stop is run by cpu stopper and it
pushes running tasks off the busiest CPU onto idle target CPU. But
there is no check to see whether target cpu is offline or not before
pushing the tasks.  With the introduction of active migration in the
scheduler tick path (see check_for_migration()) there have been
instances of attempts to migrate tasks to offline CPUs.

Add a check as to whether the target cpu is online or not to prevent
scheduling on offline CPUs.

Change-Id: Ib8ac7f8aeabd3ca7365f3eae977075952dab4f21
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>