net: neighbor: add fdb extended attribute

Add an attribute to NDA which will contain all future fdb-specific
attributes in order to avoid polluting the NDA namespace with e.g.
bridge or vxlan specific attributes. The attribute is called
NDA_FDB_EXT_ATTRS and the structure would look like:
 [NDA_FDB_EXT_ATTRS] = {
    [NFEA_xxx]
 }

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/neighbour.h

@@ -30,6 +30,7 @@ enum {
 	NDA_SRC_VNI,
 	NDA_PROTOCOL,  /* Originator of entry */
 	NDA_NH_ID,
+	NDA_FDB_EXT_ATTRS,
 	__NDA_MAX
 };
 
@@ -172,4 +173,15 @@ enum {
 };
 #define NDTA_MAX (__NDTA_MAX - 1)
 
+/* embedded into NDA_FDB_EXT_ATTRS:
+ * [NDA_FDB_EXT_ATTRS] = {
+ *     ...
+ * }
+ */
+enum {
+	NFEA_UNSPEC,
+	__NFEA_MAX
+};
+#define NFEA_MAX (__NFEA_MAX - 1)
+
 #endif