[IPv6] route: Fix prohibit and blackhole routing decision

Lookups resolving to ip6_blk_hole_entry must result in silently
discarding the packets whereas an ip6_pkt_prohibit_entry is
supposed to cause an ICMPV6_ADM_PROHIBITED message to be sent.

Thanks to Kim Nordlund <kim.nordlund@nokia.com> for noticing
this bug.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>