net: vxlan: add drop reasons support to vxlan_xmit_one()

Replace kfree_skb/dev_kfree_skb with kfree_skb_reason in vxlan_xmit_one.
No drop reasons are introduced in this commit.

The only concern of mine is replacing dev_kfree_skb with
kfree_skb_reason. The dev_kfree_skb is equal to consume_skb, and I'm not
sure if we can change it to kfree_skb here. In my option, the skb is
"dropped" here, isn't it?

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