linux-aio: prevent submitting more than MAX_EVENTS

Invoking io_setup(MAX_EVENTS) we ask kernel to create ring buffer for us
with specified number of events.  But kernel ring buffer allocation logic
is a bit tricky (ring buffer is page size aligned + some percpu allocation
are required) so eventually more than requested events number is allocated.

From a userspace side we have to follow the convention and should not try
to io_submit() more or logic, which consumes completed events, should be
changed accordingly.  The pitfall is in the following sequence:

    MAX_EVENTS = 128
    io_setup(MAX_EVENTS)

    io_submit(MAX_EVENTS)
    io_submit(MAX_EVENTS)

    /* now 256 events are in-flight */

    io_getevents(MAX_EVENTS) = 128

    /* we can handle only 128 events at once, to be sure
     * that nothing is pended the io_getevents(MAX_EVENTS)
     * call must be invoked once more or hang will happen. */

To prevent the hang or reiteration of io_getevents() call this patch
restricts the number of in-flights, which is now limited to MAX_EVENTS.

Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1468415004-31755-1-git-send-email-roman.penyaev@profitbricks.com
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: qemu-devel@nongnu.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>