io-wq: fix races around manager/worker creation and task exit

These races have always been there, they are just more apparent now that
we do early cancel of io-wq when the task exits.

Ensure that the io-wq manager sets task state correctly to not miss
wakeups for task creation. This is important if we get a wakeup after
having marked ourselves as TASK_INTERRUPTIBLE. If we do end up creating
workers, then we flip the state back to running, making the subsequent
schedule() a no-op. Also increment the wq ref count before forking the
thread, to avoid a use-after-free.

Signed-off-by: Jens Axboe <axboe@kernel.dk>