__io_req_complete() inlining is a bit weird, some compilers don't
optimise out the non-NULL branch of it even when called as
io_req_complete(). Help it a bit by extracting state and stateless
helpers out of __io_req_complete().
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>