workqueue: separate out worker_pool flags

GCWQ_MANAGE_WORKERS, GCWQ_MANAGING_WORKERS and GCWQ_HIGHPRI_PENDING
are per-pool properties.  Add worker_pool->flags and make the above
three flags per-pool flags.

The changes in this patch are mechanical and don't caues any
functional difference.  This is to prepare for multiple pools per
gcwq.

Change-Id: I1824fd1c509d8ac6b0619536621a22b15b316256
Signed-off-by: Tejun Heo <tj@kernel.org>
Git-commit: 11ebea50dbc1ade5994b2c838a096078d4c02399
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>