io_uring: keep a pointer ref_node in file_data

->cur_refs of struct fixed_file_data always points to percpu_ref
embedded into struct fixed_file_ref_node. Don't overuse container_of()
and offsetting, and point directly to fixed_file_ref_node.

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