vxlan: fix some sparse warnings

Fixes following warning:
drivers/net/vxlan.c:406:6: warning: symbol 'vxlan_fdb_free' was not declared. Should it be static?
drivers/net/vxlan.c:1111:37: warning: Using plain integer as NULL pointer

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>