xsk: Add xdp statistics to xsk_diag

Add xdp statistics to the information dumped through the xsk_diag interface

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200708072835.4427-4-ciara.loftus@intel.com