rtnetlink: fix error return code in rtnl_link_fill()

Fix to return a negative error code from the error handling case
instead of 0(possible overwrite to 0 by ops->fill_xstats call),
as returned elsewhere in this function.

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