btrfs: raid56: allow the exact minimum number of devices for balance convert

The minimum number of devices for RAID5 is 2, though this is only a bit
expensive RAID1, and for RAID6 it's 3, which is a triple copy that works
only 3 devices.

mkfs.btrfs allows that and mounting such filesystem also works, so the
conversion via balance filters is inconsistent with the others and we
should not prevent it.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -4080,11 +4080,12 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
 	allowed = BTRFS_AVAIL_ALLOC_BIT_SINGLE | BTRFS_BLOCK_GROUP_DUP;
 	if (num_devices > 1)
 		allowed |= (BTRFS_BLOCK_GROUP_RAID0 | BTRFS_BLOCK_GROUP_RAID1);
-	if (num_devices > 2)
+	if (num_devices >= 2)
 		allowed |= BTRFS_BLOCK_GROUP_RAID5;
+	if (num_devices >= 3)
+		allowed |= BTRFS_BLOCK_GROUP_RAID6;
 	if (num_devices > 3)
-		allowed |= (BTRFS_BLOCK_GROUP_RAID10 |
-			    BTRFS_BLOCK_GROUP_RAID6);
+		allowed |= BTRFS_BLOCK_GROUP_RAID10;
 	if (validate_convert_profile(&bctl->data, allowed)) {
 		int index = btrfs_bg_flags_to_raid_index(bctl->data.target);