selftests/fib_tests: Rework fib_rp_filter_test()

Currently rp_filter tests in fib_tests.sh:fib_rp_filter_test() are
failing.  ping sockets are bound to dummy1 using the "-I" option
(SO_BINDTODEVICE), but socket lookup is failing when receiving ping
replies, since the routing table thinks they belong to dummy0.

For example, suppose ping is using a SOCK_RAW socket for ICMP messages.
When receiving ping replies, in __raw_v4_lookup(), sk->sk_bound_dev_if
is 3 (dummy1), but dif (skb_rtable(skb)->rt_iif) says 2 (dummy0), so the
raw_sk_bound_dev_eq() check fails.  Similar things happen in
ping_lookup() for SOCK_DGRAM sockets.

These tests used to pass due to a bug [1] in iputils, where "ping -I"
actually did not bind ICMP message sockets to device.  The bug has been
fixed by iputils commit f455fee41c07 ("ping: also bind the ICMP socket
to the specific device") in 2016, which is why our rp_filter tests
started to fail.  See [2] .

Fixing the tests while keeping everything in one netns turns out to be
nontrivial....