This commit adds support for the new need_wakeup feature of AF_XDP. The
applications can opt-in by using the XDP_USE_NEED_WAKEUP bind() flag.
When this feature is enabled, some behavior changes:

RX side: If the Fill Ring is empty, instead of busy-polling, set the
flag to tell the application to kick the driver when it refills the Fill
Ring.

TX side: If there are pending completions or packets queued for
transmission, set the flag to tell the application that it can skip the
sendto() syscall and save time.

The performance testing was performed on a machine with the following
configuration:

- 24 cores of Intel Xeon E5-2620 v3 @ 2.40 GHz
- Mellanox ConnectX-5 Ex with 100 Gbit/s link

The results with retpoline disabled:

       | without need_wakeup  | with need_wakeup     |
       |----------------------|----------------------|
       | one core | two cores | one core | two cores |
-------|----------|-----------|----------|-----------|
txonly | 20.1     | 33.5      | 29.0     | 34.2      |
rxdrop | 0.065    | 14.1      | 12.0     | 14.1      |
l2fwd  | 0.032    | 7.3       | 6.6      | 7.2       |

"One core" means the application and NAPI run on the same core. "Two
cores" means they are pinned to different cores.
Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>