vxlan: Bypass encapsulation if the destination is local

This patch bypasses vxlan encapsulation if the destination vxlan
endpoint is a local device.

Changes since v1: added missing check for vxlan_find_vni() failure

Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>