workqueue: rename rebind_workers() to gcwq_associate() and let it handle locking...

workqueue: rename rebind_workers() to gcwq_associate() and let it handle locking and DISASSOCIATED clearing

CPU_ONLINE used to handle locking and clearing of DISASSOCIATED and
rebind_workers() just the rebinding.  This patch renames the function
to gcwq_associate() and let it handle the whole onlining.  This is for
the scheduled fix of a subtle idle worker depletion issue during
CPU_ONLINE.

Note that this removes the unnecessary relock at the end of
gcwq_associate().

This patch doesn't introduce any functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>