net: inet: diag: expose sockets cgroup classid

This is useful for directly looking up a task based on class id rather than
having to scan through all open file descriptors.

Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/inet_diag.h

@@ -142,6 +142,7 @@ enum {
 	INET_DIAG_PAD,
 	INET_DIAG_MARK,
 	INET_DIAG_BBRINFO,
+	INET_DIAG_CLASS_ID,
 	__INET_DIAG_MAX,
 };