net/af_xdp: enable custom XDP program loading

net/af_xdp: enable custom XDP program loading

The new 'xdp_prog=<string>' vdev arg allows the user to specify the path to
a custom XDP program to be set on the device, instead of the default libbpf
one. The program must have an XSK_MAP of name 'xsks_map' which will allow
for the redirection of some packets to userspace and thus the PMD, using
some criteria defined in the program. This can be useful for filtering
purposes, for example if we only want a subset of packets to reach
userspace or to drop or process a subset of packets in the kernel.

Note: a netdev may only load one program.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Tested-by: Xuekun Hu <xuekun.hu@intel.com>