xen-netback: improve ring effeciency for guest RX

xen-netback: improve ring effeciency for guest RX

There was a bug that netback routines netbk/xenvif_skb_count_slots and
netbk/xenvif_gop_frag_copy disagreed with each other, which caused
netback to push wrong number of responses to netfront, which caused
netfront to eventually crash. The bug was fixed in 6e43fc04a
("xen-netback: count number required slots for an skb more carefully").

Commit 6e43fc04a focused on backport-ability. The drawback with the
existing packing scheme is that the ring is not used effeciently, as
stated in 6e43fc04a.

skb->data like:
    |        1111|222222222222|3333        |

is arranged as:
    |1111        |222222222222|3333        |

If we can do this:
    |111122222222|22223333    |
That would save one ring slot, which improves ring effeciency.

This patch effectively reverts 6e43fc04a. That patch made count_slots
agree with gop_frag_copy, while this patch goes the other way around --
make gop_frag_copy agree with count_slots. The end result is that they
still agree with each other, and the ring is now arranged like:
    |111122222222|22223333    |

The patch that improves packing was first posted by Xi Xong and Matt
Wilson. I only rebase it on top of net-next and rewrite commit message,
so I retain all their SoBs. For more infomation about the original bug
please refer to email listed below and commit message of 6e43fc04a.

Original patch:
http://lists.xen.org/archives/html/xen-devel/2013-07/msg00760.html

Signed-off-by: Xi Xiong <xixiong@amazon.com>
Reviewed-by: Matt Wilson <msw@amazon.com>
[ msw: minor code cleanups, rewrote commit message, adjusted code
  to count RX slots instead of meta structures ]
Signed-off-by: Matt Wilson <msw@amazon.com>
Cc: Annie Li <annie.li@oracle.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
[ liuw: rebased on top of net-next tree, rewrote commit message, coding
  style cleanup. ]
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>