Embrace anonymous unions.

Several OVS structs contain embedded named unions, like this:

struct {
    ...
    union {
        ...
    } u;
};

C11 standardized a feature that many compilers already implemented
anyway, where an embedded union may be unnamed, like this:

struct {
    ...
    union {
        ...
    };
};

This is more convenient because it allows the programmer to omit "u."
in many places.  OVS already used this feature in several places.  This
commit embraces it in several others.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>
Tested-by: Alin Gabriel Serdean <aserdean@ovn.org>
Acked-by: Alin Gabriel Serdean <aserdean@ovn.org>

include/openvswitch/json.h

@@ -71,7 +71,7 @@ struct json {
         long long int integer;
         double real;
         char *string;
-    } u;
+    };
 };
 
 struct json *json_null_create(void);