vhost: cache avail index in vhost_enable_notify()

In vhost_enable_notify() we enable the notifications and we read
the avail index to check if new buffers have become available in
the meantime.

We are not caching the avail index, so when the device will call
vhost_get_vq_desc(), it will find the old value in the cache and
it will read the avail index again.

It would be better to refresh the cache every time we read avail
index, so let's change vhost_enable_notify() caching the value in
`avail_idx` and compare it with `last_avail_idx` to check if there
are new buffers available.

We don't expect a significant performance boost because
the above path is not very common, indeed vhost_enable_notify()
is often called with unlikely(), expecting that avail index has
not been updated.

We ran virtio-test/vhost-test and noticed minimal improvement as
expected. To stress the patch more, we modified vhost_test.ko to
call vhost_enable_notify()/vhost_disable_notify() on every cycle
when calling vhost_get_vq_desc(); in this case we observed a more
evident improvement, with a reduction of the test execution time
of about 3.7%.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20220121153108.187291-1-sgarzare@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>