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>