sched/core: Keep rq online after cpu isolation

Earlier this same change was introduced in 'commit a5e669b8
("sched/core: Keep rq online after cpu isolation")', but, later
'commit 78af2bab ("sched: Fix race condition with active balance")'
removed parts of code while addressing merge conflicts.
Bringing in removed code as part of this change to fix placement
issues being observed.

Change-Id: I5a13a7381f7441de139784c7f2939a7954dab85d
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>