net: ipv4: Fix NULL pointer dereference in route lookup

When calculating the multipath hash for input routes the flow info is
not available and therefore should not be used.

Fixes: 24ba1440

 ("route: Add multipath_hash in flowi_common to make user-define hash")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Cc: wenxu <wenxu@ucloud.cn>
Acked-by: wenxu <wenxu@ucloud.cn>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>