selftests: net: add delete nexthop route warning test

Add a test which causes a WARNING on kernels which treat a
nexthop route like a normal route when comparing for deletion and a
device is specified. That is, a route is found but we hit a warning while
matching it. The warning is from fib_info_nh() in include/net/nexthop.h
because we run it on a fib_info with nexthop object. The call chain is:
 inet_rtm_delroute -> fib_table_delete -> fib_nh_match (called with a
nexthop fib_info and also with fc_oif set thus calling fib_info_nh on
the fib_info and triggering the warning).

Repro steps:
 $ ip nexthop add id 12 via 172.16.1.3 dev veth1
 $ ip route add 172.16.101.1/32 nhid 12
 $ ip route delete 172.16.101.1/32 dev veth1

Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>