RDMA/mlx5: Set UDP source port based on the grh.flow_label

Calculate UDP source port based on the grh.flow_label. If grh.flow_label
is not valid, we will use minimal supported UDP source port.

Link: https://lore.kernel.org/r/20200504051935.269708-6-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>