RDMA/cma: Remove open coding of overflow checking for private_data_len

commit 8d0d2b0f upstream.

The existing tests are a little hard to comprehend. Use
check_add_overflow() instead.

Fixes: 04ded167 ("RDMA/cma: Verify private data length")
Link: https://lore.kernel.org/r/1637661978-18770-1-git-send-email-haakon.bugge@oracle.com


Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>