net/af_xdp: prefer busy polling

net/af_xdp: prefer busy polling

This commit introduces support for preferred busy polling
to the AF_XDP PMD. This feature aims to improve single-core
performance for AF_XDP sockets under heavy load.

A new vdev arg is introduced called 'busy_budget' whose default
value is 64. busy_budget is the value supplied to the kernel
with the SO_BUSY_POLL_BUDGET socket option and represents the
busy-polling NAPI budget. To set the budget to a different value
eg. 256:

--vdev=net_af_xdp0,iface=eth0,busy_budget=256

Preferred busy polling is enabled by default provided a kernel with
version >= v5.11 is in use. To disable it, set the budget to zero.

The following settings are also strongly recommended to be used in
conjunction with this feature:

echo 2 | sudo tee /sys/class/net/eth0/napi_defer_hard_irqs
echo 200000 | sudo tee /sys/class/net/eth0/gro_flush_timeout

.. where eth0 is the interface being used by the PMD.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>