ipv4: fib: Set offload indication according to nexthop flags

We're going to have capable drivers indicate route offload using the
nexthop flags, but for non-multipath routes these flags aren't dumped to
user space.

Instead, set the offload indication in the route message flags.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>