io_uring: inline io_put_req and friends

One big omission is that io_put_req() haven't been marked inline, and at
least gcc 9 doesn't inline it, not to mention that it's really hot and
extra function call is intolerable, especially when it doesn't put a
final ref.

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