rtnetlink: return the correct error code

Before this patch, function ndo_dflt_fdb_dump() will always return code
from uc fdb dump. The reture code of mc fdb dump is lost.

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>