RDMA/rxe: Use the standard method to produce udp source port

Use the standard method to produce udp source port.

Link: https://lore.kernel.org/r/20220106180359.2915060-5-yanjun.zhu@linux.dev


Signed-off-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>