btrfs: use raid_attr table in calc_stripe_length for nparity

The table is already used for ncopies, replace open coding of stripes
with the raid_attr value.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -6480,19 +6480,14 @@ static u64 calc_stripe_length(u64 type, u64 chunk_len, int num_stripes)
 {
 	int index = btrfs_bg_flags_to_raid_index(type);
 	int ncopies = btrfs_raid_array[index].ncopies;
+	const int nparity = btrfs_raid_array[index].nparity;
 	int data_stripes;
 
-	switch (type & BTRFS_BLOCK_GROUP_PROFILE_MASK) {
-	case BTRFS_BLOCK_GROUP_RAID5:
-		data_stripes = num_stripes - 1;
-		break;
-	case BTRFS_BLOCK_GROUP_RAID6:
-		data_stripes = num_stripes - 2;
-		break;
-	default:
+	if (nparity)
+		data_stripes = num_stripes - nparity;
+	else
 		data_stripes = num_stripes / ncopies;
-		break;
-	}
+
 	return div_u64(chunk_len, data_stripes);
 }