RDMA/cma: Fix accessing invalid private data for UD

If a application is using AF_IB with a UD QP, but does not provide any
private data, we will end up accessing invalid memory.  Check for this
case and handle it appropriately.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>