vxlan: dont migrate permanent fdb entries during learn

This patch fixes vxlan_snoop to not move permanent fdb entries
on learn events. This is consistent with the bridge fdb
handling of permanent entries.

Fixes: 26a41ae6 ("vxlan: only migrate dynamic FDB entries")
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>