fib: fix ip6-ll fib selection for non-ethernet interfaces

fib: fix ip6-ll fib selection for non-ethernet interfaces

Fixes case when packet to link-local address is received over
gre/mpls or other non-ethernet interface and ip6-ll fib for it
is undefined.
If by a chance ip6-ll fib index is valid, packet will be passed
to some ip6 fib with possibilities to be sent out over unrelated
interface or be looped again into ip6-link-local dpo till oom
and crash.

Type: fix
Signed-off-by: Vladislav Grishenko <themiron@yandex-team.ru>
Change-Id: Ie985f0373ea45e2926db7fb0a1ff951eca0e38f6