ipv6: Fix RTM_GETROUTE's interpretation of RTA_IIF to be consistent with ipv4

In IPv4, if an RTA_IIF attribute is specified within an RTM_GETROUTE
message, then a route is searched as if a packet was received on the
specified 'iif' interface.

However in IPv6, RTA_IIF is not interpreted in the same way:
'inet6_rtm_getroute()' always calls 'ip6_route_output()', regardless the
RTA_IIF attribute.

As a result, in IPv6 there's no way to use RTM_GETROUTE in order to look
for a route as if a packet was received on a specific interface.

Fix 'inet6_rtm_getroute()' so that RTA_IIF is interpreted as "lookup a
route as if a packet was received on the specified interface", similar
to IPv4's 'inet_rtm_getroute()' interpretation.

Reported-by: Ami Koren <amikoren@yahoo.com>
Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>