xsk: add sock_diag interface for AF_XDP

This patch adds the sock_diag interface for querying sockets from user
space. Tools like iproute2 ss(8) can use this interface to list open
AF_XDP sockets.

The user-space ABI is defined in linux/xdp_diag.h and includes netlink
request and response structs. The request can query sockets and the
response contains socket information about the rings, umems, inode and
more.

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

net/xdp/xsk.c

@@ -27,14 +27,10 @@
 
 #include "xsk_queue.h"
 #include "xdp_umem.h"
+#include "xsk.h"
 
 #define TX_BATCH_SIZE 16
 
-static struct xdp_sock *xdp_sk(struct sock *sk)
-{
-	return (struct xdp_sock *)sk;
-}
-
 bool xsk_is_setup_for_bpf_map(struct xdp_sock *xs)
 {
 	return READ_ONCE(xs->rx) &&  READ_ONCE(xs->umem) &&