vxlan_fdb_append may return error, so add the proper check,
otherwise it will cause memory leak.
Signed-off-by: NHaishuang Yan <yanhaishuang@cmss.chinamobile.com>

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