btrfs: introduce BTRFS_QGROUP_STATUS_FLAGS_MASK for later expansion

Currently we only have 3 qgroup flags:

- BTRFS_QGROUP_STATUS_FLAG_ON
- BTRFS_QGROUP_STATUS_FLAG_RESCAN
- BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT

These flags match the on-disk flags used in btrfs_qgroup_status.

But we're going to introduce extra runtime flags which will not reach
disks.

So here we introduce a new mask, BTRFS_QGROUP_STATUS_FLAGS_MASK, to
make sure only those flags can reach disks.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>