vxlan: fix a shadow local variable

Commit 79b16aad
("udp_tunnel: Pass UDP socket down through udp_tunnel{, 6}_xmit_skb()")
introduce 'sk' but we already have one inner 'sk'.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>