Replace io_uring workqueues with io-wq

Most of the justification for this work is done in the commit messages
themselves, but the tldr is that there are various bits of functionality
that io_uring needs that workqueues don't (and can't) provide. Hence
this adds a small replacement thread pool implementation that caters to
the needs of io_uring, both current and future ones.

Hopefully the sched core changes are palatable. io-wq uses the same
sched in/out hooks as workqueue, and if the task isn't either a
workqueue or io-wq worker, there should be no extra overhead there.

Patches are on top of my for-5.5/io_uring branch, and can also be found
here:

http://git.kernel.dk/cgit/linux-block/log/?h=for-5.5/io_u...

This passes io_uring IO testing and repeated runs of the liburing
regressions suite, and passes tests that would previously deadlock.

 fs/Kconfig            |   3 +
 fs/Makefile           |   1 +
 fs/io-wq.c            | 790 ++++++++++++++++++++++++++++++++++++++++++
 fs/io-wq.h            |  55 +++
 fs/io_uring.c         | 402 +++++----------------
 include/linux/sched.h |   1 +
 init/Kconfig          |   1 +
 kernel/sched/core.c   |  16 +-
 8 files changed, 948 insertions(+), 321 deletions(-)

-- 
Jens Axboe