virtio_net: fix ndo_xdp_xmit crash towards dev not ready for XDP

When a driver implements the ndo_xdp_xmit() function, there is
(currently) no generic way to determine whether it is safe to call.

It is e.g. unsafe to call the drivers ndo_xdp_xmit, if it have not
allocated the needed XDP TX queues yet.  This is the case for
virtio_net, which first allocates the XDP TX queues once an XDP/bpf
prog is attached (in virtnet_xdp_set()).

Thus, a crash will occur for virtio_net when redirecting to another
virtio_net device's ndo_xdp_xmit, which have not attached a XDP prog.
The sample xdp_redirect_map tries to attach a dummy XDP prog to take
this into account, but it can also easily fail if the virtio_net (or
actually underlying vhost driver) have not allocated enough extra
queues for the device.

Allocating more queue this is currently a manual config.
Hint for libvirt XML add:

  <driver name='vhost' queues='16'>
    <host mrg_rxbuf='off'/>
    <guest tso4='off' tso6='off' ecn='off' ufo='off'/>
  </driver>

The solution in this patch is to check that the device have loaded an
XDP/bpf prog before proceeding.  This is similar to the check
performed in driver ixgbe.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>