IB/core: Avoid a potential OOPs for an unused optional parameter

The ev_file is an optional parameter for CQ creation. If the parameter
is not passed, the ev_file pointer will be NULL.  Using that pointer
to set the cq_context will result in an OOPs.

Verify that ev_file is not NULL before using.

Cc: <stable@vger.kernel.org> # 4.14.x
Fixes: 9ee79fce ("IB/core: Add completion queue (cq) object actions")
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/uverbs_std_types.c

@@ -316,7 +316,7 @@ static int uverbs_create_cq_handler(struct ib_device *ib_dev,
 	cq->uobject       = &obj->uobject;
 	cq->comp_handler  = ib_uverbs_comp_handler;
 	cq->event_handler = ib_uverbs_cq_event_handler;
-	cq->cq_context    = &ev_file->ev_queue;
+	cq->cq_context    = ev_file ? &ev_file->ev_queue : NULL;
 	obj->uobject.object = cq;
 	obj->uobject.user_handle = user_handle;
 	atomic_set(&cq->usecnt, 0);