drm/amdkfd: Fix handling of return code of dma_buf_get

On errors, dma_buf_get returns a negative error code, rather than NULL.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdkfd/kfd_chardev.c

@@ -1629,8 +1629,8 @@ static int kfd_ioctl_import_dmabuf(struct file *filep,
 		return -EINVAL;
 
 	dmabuf = dma_buf_get(args->dmabuf_fd);
-	if (!dmabuf)
-		return -EINVAL;
+	if (IS_ERR(dmabuf))
+		return PTR_ERR(dmabuf);
 
 	mutex_lock(&p->mutex);