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

Replace kfree_skb with kfree_skb_reason in vxlan_encap_bypass, and no new
skb drop reason is added in this commit.

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