shadowsocks-libev: ignore error from flushing route table

shadowsocks-libev: ignore error from flushing route table

Linux kernel and iproute2 together now implement strict checking of the
existence of route tables.

Previously kernel does not support filtering by table id, now it does
and will error with nlmsgerr "ipv4: FIB table does not exist".

Previously iproute2 dump all routes and filter by table id in userspace,
now this has changed with iproute2 commit c7e6371bc4af ("ip route: Add
protocol, table id and device to dump request")

Error scene

	root@OpenWrt:/# ip route flush table 100
	Error: ipv4: FIB table does not exist.
	Flush terminated
	root@OpenWrt:/# echo $?
	2

Fixes: https://github.com/openwrt/packages/issues/12095
Ref: https://lists.openwall.net/netdev/2019/05/02/105
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>