gpu: ion: Take handle reference in ion_vma_open

When ion_vma_open is called, a reference to the handle in
the vma must be taken. Otherwise, if forking occurs,
ion_vma_close will be called twice which will leave one of
the calls with an invalid reference.

drivers/gpu/ion/ion.c

@@ -798,6 +798,7 @@ static void ion_vma_open(struct vm_area_struct *vma)
 		vma->vm_private_data = NULL;
 		return;
 	}
+	ion_handle_get(handle);
 	pr_debug("%s: %d client_cnt %d handle_cnt %d alloc_cnt %d\n",
 		 __func__, __LINE__,
 		 atomic_read(&client->ref.refcount),