workqueue: Fix workqueue stall issue after cpu down failure

When the hotplug notifier call chain with CPU_DOWN_PREPARE
is broken before reaching workqueue_cpu_down_callback(),
rebind_workers() adds WORKER_REBOUND flag for running workers.
Hence, the nr_running of the pool is not increased when scheduler
wakes up the worker. The fix is skipping adding WORKER_REBOUND
flag when the worker doesn't have WORKER_UNBOUND flag in
CPU_DOWN_FAILED path.

Change-Id: I2528e9154f4913d9ec14b63adbcbcd1eaa8a8452
Signed-off-by: Se Wang (Patrick) Oh <sewango@codeaurora.org>