io_uring: compact SQ/CQ heads/tails

Queues heads and tails cache line aligned. That makes sq, cq taking 4
lines or 5 lines if we include the rest of struct io_rings (e.g.
sq_flags is frequently accessed).

Since modern io_uring is mostly single threaded, it doesn't make much
send to spread them as such, it wastes space and puts additional pressure
on caches. Put them all into a single line.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9c8deddf9a7ed32069235a530d1e117fb460bc4c.1692916914.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/io_uring_types.h

@@ -69,8 +69,8 @@ struct io_uring_task {
 };
 
 struct io_uring {
-	u32 head ____cacheline_aligned_in_smp;
-	u32 tail ____cacheline_aligned_in_smp;
+	u32 head;
+	u32 tail;
 };
 
 /*