vxlan: do not age static remote mac entries

Mac aging is applicable only for dynamically learnt remote mac
entries. Check for user configured static remote mac entries
and skip aging.

Signed-off-by: Balakrishnan Raman <ramanb@cumulusnetworks.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>