selftest: Fixes for icmp_redirect test

I was really surprised that the IPv6 mtu exception followed by redirect
test was passing as nothing about the code suggests it should. The problem
is actually with the logic in the test script.

Fix the test cases as follows:
1. add debug function to dump the initial and redirect gateway addresses
   for ipv6. This is shown only in verbose mode. It helps verify the
   output of 'route get'.

2. fix the check_exception logic for the reset case to make sure that
   for IPv4 neither mtu nor redirect appears in the 'route get' output.
   For IPv6, make sure mtu is not present and the gateway is the initial
   R1 lladdr.

3. fix the reset logic by using a function to delete the routes added by
   initial_route_*. This format works better for the nexthop version of
   the tests.

While improving the test cases, go ahead and ensure that forwarding is
disabled since IPv6 redirect requires it.

Also, runs with kernel debugging enabled sometimes show a failure with
one of the ipv4 tests, so spread the pings over longer time interval.

The end result is that 2 tests now show failures:

TEST: IPv6: mtu exception plus redirect                    [FAIL]

and the VRF version.

This is a bug in the IPv6 logic that will need to be fixed
separately. Redirect followed by MTU works because __ip6_rt_update_pmtu
hits the 'if (!rt6_cache_allowed_for_pmtu(rt6))' path and updates the
mtu on the exception rt6_info.

MTU followed by redirect does not have this logic. rt6_do_redirect
creates a new exception and then rt6_insert_exception removes the old
one which has the MTU exception.

Fixes: ec810535 ("selftests: Add redirect tests")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>