vxlan: only migrate dynamic FDB entries

Only migrate dynamic forwarding table entries, don't modify
static entries. If packet received from incorrect source IP address
assume it is an imposter and drop it.

This patch applies only to -net, a different patch would be needed for earlier
kernels since the NTF_SELF flag was introduced with 3.10.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>