io_uring: split ref_node alloc and init

A simple prep patch allowing to set refnode callbacks after it was
allocated. This needed to 1) keep ourself off of hi-level functions
where it's not pretty and they are not necessary 2) amortise ref_node
allocation in the future, e.g. for updates.

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