RDMA/cma: Initialize the flow label of CM's route path record

If flow label is not set by the user or it's not IPv4, initialize it with
the cma src/dst based on the "Kernighan and Ritchie's hash function".

Link: https://lore.kernel.org/r/20200504051935.269708-5-leon@kernel.org
Signed-off-by: Mark Zhang <markz@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>