net: Do not call fib6_info_hw_flags_set() when IPv6 is disabled

With the next patch mlxsw and netdevsim will fail in compilation if
CONFIG_IPV6 is disabled.

Do not call fib6_info_hw_flags_set() when IPv6 is disabled.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>