ip route: restore_handler should check tb[RTA_PREFSRC] for local networks

ip route: restore_handler should check tb[RTA_PREFSRC] for local networks

Prior to this patch, If one route entry's RTA_PREFSRC and RTA_GATEWAY
both were NULL, it was supposed to be restored ONLY as a local address.

But as it didn't check tb[RTA_PREFSRC] when restoring local networks,
rtattr_cmp would return a success if it was NULL, this route entry would
be restored again as a local network.

This patch is to add tb[RTA_PREFSRC] check when restoring local networks.

Fixes: 74af8dd9620e ("ip route: restore route entries in correct order")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Tested-by: Phil Sutter <phil@nwl.cc>