io_uring: remove dependency on ring->sq/cq_entries

We have numbers of {sq,cq} entries cached in ctx, don't look up them in
user-shared rings as 1) it may fetch additional cacheline 2) user may
change it and so it's always error prone.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/745d31bc2da41283ddd0489ef784af5c8d6310e9.1621201931.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>