RDMA/iwcm: Don't touch cmid after dropping reference

The function cm_work_handler() cannot touch the cm_id after it derefs
it, because it might be freed on another concurrent thread.  If there
are more work items queued for this cm_id, then we know there must be
more references because they are added when the work items are queued.
So in the while loop inside cm_work_handler(), after derefing, if the
queue is empty, then exit the function.  Otherwise we know it's safe
to re-acquire the lock.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>