netifd: correctly handle source-parameter for IPv4 routes

Thanks to Kristian Evensen

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43400 3c298f89-4303-0410-b956-a3cf2f4a3e73
Merge branch 'master' of git://git.openwrt.org/openwrt