io_uring: fix race condition reading SQ entries

A read memory barrier is required between reading SQ tail and reading
the actual data belonging to the SQ entry.

Userspace needs a matching write barrier between writing SQ entries and
updating SQ tail (using smp_store_release to update tail will do).

Signed-off-by: Stefan Bühler <source@stbuehler.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>