samples/bpf: convert xdpsock to use libbpf for AF_XDP access

This commit converts the xdpsock sample application to use the AF_XDP
functions present in libbpf. This cuts down the size of it by nearly
300 lines of code.

The default ring sizes plus the batch size has been increased and the
size of the umem area has decreased. This so that the sample application
will provide higher throughput. Note also that the shared umem code
has been removed from the sample as this is not supported by libbpf
at this point in time.

Tested-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>