btrfs: Use explicit round_down call rather than open-coding it

No functional changes.
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -4762,7 +4762,7 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
 	     btrfs_cmp_device_info, NULL);
 
 	/* round down to number of usable stripes */
-	ndevs -= ndevs % devs_increment;
+	ndevs = round_down(ndevs, devs_increment);
 
 	if (ndevs < devs_increment * sub_stripes || ndevs < devs_min) {
 		ret = -ENOSPC;