macvtap: do not add self to waitqueue if doing a nonblock read

There's no need to add self to waitqueue if doing a nonblock read. This could
help to avoid the spinlock contention.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>