selftests: pmtu: Make list_flush_ipv6_exception test more demanding

Instead of just listing and flushing two cached exceptions, create
a relatively big number of them, and count how many are listed. Single
netlink dump messages contain approximately 25 entries each, and this
way we can make sure the partial dump tracking mechanism is working
properly.

While at it, also ensure that no cached routes can be listed after
flush, and remove 'sleep 1' calls, they are not actually needed.

v7: No changes

v6:
  - Merge this patch into series including fix, as it's also targeted
    for net-next. No actual changes

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>