vxlan: fix potential NULL dereference in arp_reduce()

This patch fixes a NULL pointer dereference in the event of an
skb allocation failure in arp_reduce().

Signed-Off-By: David L Stevens <dlstevens@us.ibm.com>
Acked-by: Cong Wang <cwang@twopensource.com>

Signed-off-by: David S. Miller <davem@davemloft.net>