vxlan: fix a free after use

pskb_may_pull maybe change skb->data and make eth pointer oboslete,
so eth needs to reload

Fixes: 91269e39 ("vxlan: using pskb_may_pull as early as possible")
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>