ss: add AF_XDP support

ss: add AF_XDP support

AF_XDP is an address family that is optimized for high performance
packet processing.

This patch adds AF_XDP support to ss(8) so that sockets can be queried
and monitored.

Example:
$ sudo ss --xdp -e -p -m
Recv-Q      Send-Q           Local Address:Port             Peer Address:Port

0           0                   enp134s0f0:q20                          *
 users:(("xdpsock",pid=17787,fd=3)) ino:39424 sk:4
        rx(entries:2048)
        tx(entries:2048)
        umem(id:1,size:8388608,num_pages:2048,chunk_size:2048,headroom:0,ifindex:7,
qid:20,zc:0,refs:1)
        fr(entries:2048)
        cr(entries:2048) skmem:(r0,rb212992,t0,tb212992,f0,w0,o0,bl0,d0)
0           0                    enp24s0f0:q0                           *
 users:(("xdpsock",pid=17780,fd=3)) ino:37384 sk:5
        rx(entries:2048)
        tx(entries:2048)
        umem(id:0,size:8388608,num_pages:2048,chunk_size:2048,headroom:0,ifindex:6,
qid:0,zc:1,refs:1)
        fr(entries:2048)
        cr(entries:2048) skmem:(r0,rb212992,t0,tb212992,f0,w0,o0,bl0,d0)

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: David Ahern <dsahern@gmail.com>