Fix infinite loop on platforms with severely limited aio resources

Fix infinite loop on platforms with severely limited aio resources

Problem hit on OSX, where with 4 jobs each with a queue depth larger
than what the system supports, you can get into a situation where
any given process can get EAGAIN on queueing AIO even if it has
nothing queued already. Check for this condition to avoid fio's
IO loop going into a death spiral.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>