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

Replace kfree_skb() with kfree_skb_reason() in vxlan_mdb_xmit. No drop
reasons are introduced 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>