io-wq: get rid of FIXED worker flag

It makes the logic easier to follow if we just get rid of the fixed worker
flag, and simply ensure that we never exit the last worker in the group.
This also means that no particular worker is special.

Just track the last timeout state, and if we have hit it and no work
is pending, check if there are other workers. If yes, then we can exit
this one safely.

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