io_uring: fix miscounting ios_left

io_req_init() doesn't decrement state->ios_left if a request doesn't
need ->file, it just returns before that on if(!needs_file). That's
not really a problem but may cause overhead for an additional fput().
Also inline and kill io_req_set_file() as it's of no use anymore.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>