net: add fdb generic dump routine

This adds a generic dump routine drivers can call. It
should be sufficient to handle any bridging model that
uses the unicast address list. This should be most SR-IOV
enabled NICs.

v2: return error on nlmsg_put and use -EMSGSIZE instead
    of -ENOMEM this is inline other usages

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>