ipv4: Set nexthop flags in a more consistent way

Be more consistent about the way in which the nexthop flags are set and
set them in one go.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20201110102553.1924232-1-idosch@idosch.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>