VSOCK: add sock_diag interface

This patch adds the sock_diag interface for querying sockets from
userspace.  Tools like ss(8) and netstat(8) can use this interface to
list open sockets.

The userspace ABI is defined in <linux/vm_sockets_diag.h> and includes
netlink request and response structs.  The request can query sockets
based on their sk_state (e.g. listening sockets only) and the response
contains socket information fields including the local/remote addresses,
inode number, etc.

This patch does not dump VMCI pending sockets because I have only tested
the virtio transport, which does not use pending sockets.  Support can
be added later by extending vsock_diag_dump() if needed by VMCI users.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>