selftests: Add test with multiple prefixes using single nexthop

Add tests where multiple FIB entries use the same nexthop object. Generate
per-cpu cached routes for each by running ping on each cpu, and then
generate exceptions unique to each prefix (remote host) with different
mtus.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>