aio: use the macro rather than the inline magic number

Replace the inline magic number with the ready-made macro(AIO_RING_MAGIC),
just clean up.

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>