io_uring/register: limit ring resizing to DEFER_TASKRUN

With DEFER_TASKRUN, we know the ring can't be both waited upon and
resized at the same time. This is important for CQ resizing. Allowing SQ
ring resizing is more trivial, but isn't the interesting use case. Hence
limit ring resizing in general to DEFER_TASKRUN only for now. This isn't
a huge problem as CQ ring resizing is generally the most useful on
networking type of workloads where it can be hard to size the ring
appropriately upfront, and those should be using DEFER_TASKRUN for
better performance.

Fixes: 79cfe9e5 ("io_uring/register: add IORING_REGISTER_RESIZE_RINGS")
Signed-off-by: Jens Axboe <axboe@kernel.dk>