fib6_rte_to_nh_*: return a link-local gw address with scope embedded

In fib6_rte_to_nh_* when returning a link-local gateway address
currently we do clear the scope. That could be recovered using
the ifp returned as well, but the code in general seems to
expect a link-local address with scope embeedded as otherwise
the "dst" (gw) passed to the output routines will not include
scope and not send the packet out (the right interface).

Do not clear the scope when returning a link-local address and
allow packets to go out (the right interface).

Remove the (now) extra scope recovery in the IPv6 fast-fwd code.

Sponsored by: Netflix
Reviewed by: melifaro, ae
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D23872