io_uring: replace smp_mb() with smp_mb__after_atomic() in io_sq_thread()

The IORING_SQ_NEED_WAKEUP flag is now set using atomic_or() which
implies a full barrier on some architectures but it is not required to
do so. Use the more appropriate smp_mb__after_atomic() which avoids the
extra barrier on those architectures.
Signed-off-by: Almog Khaikin <almogkh@gmail.com>
Link: https://lore.kernel.org/r/20220426163403.112692-1-almogkh@gmail.com
Fixes: 8018823e6987 ("io_uring: serialize ctx->rings->sq_flags with atomic_or/and")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -8229,7 +8229,7 @@ static int io_sq_thread(void *data)
 				 * Ensure the store of the wakeup flag is not
 				 * reordered with the load of the SQ tail
 				 */
-				smp_mb();
+				smp_mb__after_atomic();
 
 				if (io_sqring_entries(ctx)) {
 					needs_sched = false;