virtio-blk: fixup coccinelle warnings

coccicheck complains about the use of snprintf() in sysfs show
functions:
WARNING use scnprintf or sprintf

Use sysfs_emit instead of scnprintf or sprintf makes more sense.

Reported-by: Zeal Robot <[e-mail]>
Signed-off-by: Ye Guojin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Michael S. Tsirkin <[e-mail]>
Reviewed-by: Stefan Hajnoczi <[e-mail]>
Reviewed-by: Stefano Garzarella <[e-mail]>

drivers/block/virtio_blk.c

···
 	u8 writeback = virtblk_get_cache_mode(vblk->vdev);
 
 	BUG_ON(writeback >= ARRAY_SIZE(virtblk_cache_types));
-	return snprintf(buf, 40, "%s\n", virtblk_cache_types[writeback]);
+	return sysfs_emit(buf, "%s\n", virtblk_cache_types[writeback]);
 }
 
 static DEVICE_ATTR_RW(cache_type);