IB/cma: allocating too much memory in make_cma_ports()

The issue here is that there is a cut and paste bug.  When we allocate
cma_dev_group->default_ports_group we use "sizeof(*cma_dev_group->ports)"
instead of "sizeof(*cma_dev_group->default_ports_group)".

We're bumping up against the 80 character limit so I introduced a new
local pointer "ports_group" to get around that.

Fixes: 045959db ('IB/cma: Add configfs for rdma_cm')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>