block: sunvdc: add check for mdesc_grab() returning NULL

In vdc_port_probe(), we should check the return value of mdesc_grab() as
it may return NULL, which can cause potential NPD bug.

Fixes: 43fdf27 ("[SPARC64]: Abstract out mdesc accesses for better MD update handling.")
Signed-off-by: Liang He <windhl@126.com>
Link: https://lore.kernel.org/r/20230315062032.1741692-1-windhl@126.com
[axboe: style cleanup]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/sunvdc.c

@@ -972,6 +972,8 @@ static int vdc_port_probe(struct vio_dev *vdev, const struct vio_device_id *id)
 	print_version();
 
 	hp = mdesc_grab();
+	if (!hp)
+		return -ENODEV;
 
 	err = -ENODEV;
 	if ((vdev->dev_no << PARTITION_SHIFT) & ~(u64)MINORMASK) {