net: vxlan: add skb drop reasons to vxlan_rcv()

Introduce skb drop reasons to the function vxlan_rcv(). Following new
drop reasons are added:

  SKB_DROP_REASON_VXLAN_INVALID_HDR
  SKB_DROP_REASON_VXLAN_VNI_NOT_FOUND
  SKB_DROP_REASON_IP_TUNNEL_ECN

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>