vxlan: handle skb_clone failure

If skb_clone fails if out of memory then just skip the fanout.

Problem was introduced in 3.10 with:
  commit 6681712d
  Author: David Stevens <dlstevens@us.ibm.com>
  Date:   Fri Mar 15 04:35:51 2013 +0000

    vxlan: generalize forwarding tables

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>