[IB] uverbs: Avoid NULL pointer deref on CQ async event

Userspace CQs that have no completion event channel attached end up
with their cq_context set to NULL.  However, asynchronous events like
"CQ overrun" can still occur on such CQs, so add a uverbs_file member
to struct ib_ucq_object that we can follow to deliver these events.
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/core/uverbs.h

@@ -113,6 +113,7 @@ struct ib_uevent_object {
 
 struct ib_ucq_object {
 	struct ib_uobject	uobject;
+	struct ib_uverbs_file  *uverbs_file;
 	struct list_head	comp_list;
 	struct list_head	async_list;
 	u32			comp_events_reported;

drivers/infiniband/core/uverbs_cmd.c

@@ -602,6 +602,7 @@ ssize_t ib_uverbs_create_cq(struct ib_uverbs_file *file,
 
 	uobj->uobject.user_handle   = cmd.user_handle;
 	uobj->uobject.context       = file->ucontext;
+	uobj->uverbs_file	    = file;
 	uobj->comp_events_reported  = 0;
 	uobj->async_events_reported = 0;
 	INIT_LIST_HEAD(&uobj->comp_list);

drivers/infiniband/core/uverbs_main.c

@@ -442,13 +442,10 @@ static void ib_uverbs_async_handler(struct ib_uverbs_file *file,
 
 void ib_uverbs_cq_event_handler(struct ib_event *event, void *context_ptr)
 {
-	struct ib_uverbs_event_file *ev_file = context_ptr;
-	struct ib_ucq_object *uobj;
-
-	uobj = container_of(event->element.cq->uobject,
+	struct ib_ucq_object *uobj = container_of(event->element.cq->uobject,
 						  struct ib_ucq_object, uobject);
 
-	ib_uverbs_async_handler(ev_file->uverbs_file, uobj->uobject.user_handle,
+	ib_uverbs_async_handler(uobj->uverbs_file, uobj->uobject.user_handle,
 				event->event, &uobj->async_list,
 				&uobj->async_events_reported);