RDMA/cma: Protect against adding device during destruction

Closes a window where address resolution can attach an rdma_cm_id to a
device during destruction of the rdma_cm_id.  This can result in the
rdma_cm_id remaining in the device list after its memory has been
freed.

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