ipv6: drop metadata dst in ip6_route_input

The fix in commit 48fb6b55 is incomplete, as now ip6_route_input can be
called with non-NULL dst if it's a metadata dst and the reference is leaked.
Drop the reference.

Fixes: 48fb6b55 ("ipv6: fix crash over flow-based vxlan device")
Fixes: ee122c79 ("vxlan: Flow based tunneling")
CC: Wei-Chun Chao <weichunc@plumgrid.com>
CC: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>