RFC virtio and vhost-net capacity handling

OK, here's a new attempt to use the new capacity api. I also added more comments to clarify the logic. Hope this is more readable. Let me know pls.

This is on top of the patches applied by Rusty.

Note: there are now actually 2 calls to fee_old_xmit_skbs on data path so instead of passing flag '2' to the
second one I thought we can just make each call free up
at least 1 skb. This will work and even might be a bit faster (less branches), but in the end I discarded this idea as too fragile (relies on two calls on data path to function properly).

Warning: untested. Posting now to give people chance to comment on the API.

Michael S. Tsirkin (3):
virtio_ring: add capacity check API
virtio_net: fix tx capacity checks using new API
virtio_net: limit xmit polling

 drivers/net/virtio_net.c     |   65 +++++++++++++++++++++++------------------
 drivers/virtio/virtio_ring.c |    8 +++++
 include/linux/virtio.h       |    5 +++
 3 files changed, 49 insertions(+), 29 deletions(-)

1.7.5.53.gc233e