net/af_xdp: re-enable secondary process support

net/af_xdp: re-enable secondary process support

Secondary process support had been disabled for the AF_XDP PMD because
there was no logic in place to share the AF_XDP socket file descriptors
between the processes. This commit introduces this logic using the IPC
APIs.

Rx and Tx are disabled in the secondary process due to memory mapping of
the AF_XDP rings being assigned by the kernel in the primary process only.
However other operations including retrieval of stats are permitted.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>