vxlan: don't flush static fdb entries on admin down

This patch skips flushing static fdb entries in
ndo_stop, but flushes all fdb entries during vxlan
device delete. This is consistent with the bridge
driver fdb

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