vxlan: support NTF_USE refresh of fdb entries

This makes use of NTF_USE in vxlan driver consistent
with bridge driver.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>