io_uring: pre-calculate scq layout

io_uring: pre-calculate scq layout

Move ring layouts calculations into io_prepare_config(), so that more
misconfiguration checking can be done earlier before creating a ctx.
It also deduplicates some code with ring resizing. And as a bonus, now
it initialises params->sq_off.array, which is closer to all other user
offset init, and also applies it to ring resizing, which was previously
missing it.

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