workqueue: separate out init_rescuer()

Separate out init_rescuer() from __alloc_workqueue_key() to prepare
for early init support for WQ_MEM_RECLAIM.  This patch doesn't
introduce any functional changes.

Change-Id: Id5e110d72f21a25dc5f0f33b395040d51de49fcf
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com
Git-commit: 983c751532738b83c4c5b51192ebc6fbfe9330f7
Git-Repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/


Signed-off-by: Prateek Sood <prsood@codeaurora.org>