sched: Don't update idle_cpus_mask during core isolation

Remove code to update nohz.idle_cpus_mask from
do_isolation_work_cpu_stop(), instead update find_new_ilb() to exclude
isolated_cpus. This is done to fix the issue where cpu is not present
in nohz.idle_cpus_mask but NOHZ_TICK_STOPPED is set. As a result even
if the cpu is unisolated, task won't see the cpu as idle due to
inconsistent nohz.idle_cpus_mask.
Also restore nohz_balance_exit_idle() to upstream version.

Change-Id: Ia0dfa18ca9d55512c461801234de889a173e13da
Signed-off-by: Puja Gupta <pujag@codeaurora.org>
[markivx: Trivial conflict due to asym code in nohz_needed]
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>
[satyap@codeaurora.org: trivial merge conflict resolution]
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>