kthread: add a global worker thread.

[ Upstream commit 0532e87d9d44795221aa921ba7024bde689cc894 ]

Add kthread_schedule_work() which uses a global kthread for all its
jobs.
Split the cgroup include to avoid recussive includes from interrupt.h.
Fixup everything that fails to build (and did not include all header).

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
[
  Fixed up include in blk-cgroup.h reported by Juri Lelli
  http://lkml.kernel.org/r/20190722083009.GE25636@localhost.localdomain


]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>