RDMA/ucma: Add .nodename/.mode to tell userspace where to create device node

We want udev to create a device node under /dev/infiniband with
permission 0666 for rdma_cm, so add that info to our struct miscdevice.

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