btrfs: add support for 3-copy replication (raid1c3)

Add new block group profile to store 3 copies in a simliar way that
current RAID1 does. The profile attributes and constraints are defined
in the raid table and used by the same code that already handles the
2-copy RAID1.

The minimum number of devices is 3, the maximum number of devices/chunks
that can be lost/damaged is 2. Like RAID6 but with 33% space
utilization.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/super.c

@@ -1935,6 +1935,8 @@ static inline int btrfs_calc_avail_data_space(struct btrfs_fs_info *fs_info,
 		num_stripes = nr_devices;
 	else if (type & BTRFS_BLOCK_GROUP_RAID1)
 		num_stripes = 2;
+	else if (type & BTRFS_BLOCK_GROUP_RAID1C3)
+		num_stripes = 3;
 	else if (type & BTRFS_BLOCK_GROUP_RAID10)
 		num_stripes = 4;