aio: Fix an error code in __io_submit_one()

This accidentally returns the wrong variable.  The "req->ki_eventfd"
pointer is NULL so this return success.

Fixes: 7316b49c ("aio: move sanity checks and request allocation to io_submit_one()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>