aio: double aio_max_nr in calculations

With the changes to use percpu counters for aio event ring size calculation,
existing increases to aio_max_nr are now insufficient to allow for the
allocation of enough events.  Double the value used for aio_max_nr to account
for the doubling introduced by the percpu slack.

Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>