bridge: add MDB get uAPI attributes

Add MDB get attributes that correspond to the MDB set attributes used in
RTM_NEWMDB messages. Specifically, add 'MDBA_GET_ENTRY' which will hold
a 'struct br_mdb_entry' and 'MDBA_GET_ENTRY_ATTRS' which will hold
'MDBE_ATTR_*' attributes that are used as indexes (source IP and source
VNI).

An example request will look as follows:

[ struct nlmsghdr ]
[ struct br_port_msg ]
[ MDBA_GET_ENTRY ]
	struct br_mdb_entry
[ MDBA_GET_ENTRY_ATTRS ]
	[ MDBE_ATTR_SOURCE ]
		struct in_addr / struct in6_addr
	[ MDBE_ATTR_SRC_VNI ]
		u32
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/if_bridge.h

@@ -723,6 +723,24 @@ enum {
 };
 #define MDBA_SET_ENTRY_MAX (__MDBA_SET_ENTRY_MAX - 1)
 
+/* [MDBA_GET_ENTRY] = {
+ *    struct br_mdb_entry
+ *    [MDBA_GET_ENTRY_ATTRS] = {
+ *       [MDBE_ATTR_SOURCE]
+ *          struct in_addr / struct in6_addr
+ *       [MDBE_ATTR_SRC_VNI]
+ *          u32
+ *    }
+ * }
+ */
+enum {
+	MDBA_GET_ENTRY_UNSPEC,
+	MDBA_GET_ENTRY,
+	MDBA_GET_ENTRY_ATTRS,
+	__MDBA_GET_ENTRY_MAX,
+};
+#define MDBA_GET_ENTRY_MAX (__MDBA_GET_ENTRY_MAX - 1)
+
 /* [MDBA_SET_ENTRY_ATTRS] = {
  *    [MDBE_ATTR_xxx]
  *    ...