vxlan: fix a potential issue when create a new vxlan fdb entry.

vxlan_fdb_append may return error, so add the proper check,
otherwise it will cause memory leak.

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>

Changes in v2:
  - Unnecessary to initialize rc to zero.
Acked-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>