aio: Skip timer for io_getevents if timeout=0

aio: Skip timer for io_getevents if timeout=0

In this case, it is basically a polling. Let's not involve timer at all
because that would hurt performance for application event loops.

In an arbitrary test I've done, io_getevents syscall elapsed time
reduces from 50000+ nanoseconds to a few hundereds.

Signed-off-by: Fam Zheng <[email protected]>
Signed-off-by: Benjamin LaHaise <[email protected]>