netinet: handle blackhole routes

If during ip_forward() we find a blackhole (or reject) route we should stop
processing and count this in the 'cantforward' counter, just like we already do
for IPv6.
Blackhole routes are set to use the loopback interface, so we don't actually
incorrectly forward traffic, but we do fail to count it as unroutable.

Test this, both for IPv4 and IPv6.

Reviewed by: melifaro
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D47529