RDMA: Directly cast the sockaddr union to sockaddr

gcc 9 now does allocation size tracking and thinks that passing the member
of a union and then accessing beyond that member's bounds is an overflow.

Instead of using the union member, use the entire union with a cast to
get to the sockaddr. gcc will now know that the memory extends the full
size of the union.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>