vxlan: Fix double free of skb.

In case of error vxlan_xmit_one() can free already freed skb.
Also fixes memory leak of dst-entry.

Fixes: acbf74a7 ("vxlan: Refactor vxlan driver to make use
of the common UDP tunnel functions").

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>