RDMA/cma: Use correct size when writing netlink stats

The code was using the src size when formatting the dst. They are almost
certainly the same value but it reads wrong.

Fixes: ce117ffa ("RDMA/cma: Export AF_IB statistics")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>