[RFC PATCH V2] New Xen netback implementation

[Wei Liu <wei.liu2@citrix.com>]

A new netback implementation which includes three major features:

 - Global page pool support
 - NAPI + kthread 1:1 model
 - Netback internal name changes

Changes in V2:
 - Fix minor bugs in V1
 - Embed pending_tx_info into page pool
 - Per-cpu scratch space
 - Notification code path clean up

This patch series is the foundation of furture work. So it is better
to get it right first. Patch 1 and 3 have the real meat.

The first benifit of 1:1 model will be scheduling fairness.

The rational behind a global page pool is that we need to limit
overall memory consumed by all vifs.

Utilization of NAPI enables the possibility to mitigate
interrupts/events, the code path is cleaned up in a separated patch.

Netback internal changes cleans up the code structure after switching
to 1:1 model. It also prepares netback for further code layout
changes.

---
 drivers/net/xen-netback/Makefile    |    2 +-
 drivers/net/xen-netback/common.h    |   78 ++--
 drivers/net/xen-netback/interface.c |  117 ++++--
 drivers/net/xen-netback/netback.c   |  836 ++++++++++++++---------------------
 drivers/net/xen-netback/page_pool.c |  185 ++++++++
 drivers/net/xen-netback/page_pool.h |   66 +++
 drivers/net/xen-netback/xenbus.c    |    6 +-
 7 files changed, 704 insertions(+), 586 deletions(-)