aio: fix spectre gadget in lookup_ioctx

Matthew pointed out that the ioctx_table is susceptible to spectre v1,
because the index can be controlled by an attacker.  The below patch
should mitigate the attack for all of the aio system calls.

Cc: stable@vger.kernel.org
Reported-by: Matthew Wilcox <willy@infradead.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>