net: vxlan: use kfree_skb_reason() in vxlan_xmit()

Replace kfree_skb() with kfree_skb_reason() in vxlan_xmit(). Following
new skb drop reasons are introduced for vxlan:

/* no remote found for xmit */
SKB_DROP_REASON_VXLAN_NO_REMOTE
/* packet without necessary metadata reached a device which is
 * in "external" mode
 */
SKB_DROP_REASON_TUNNEL_TXINFO

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