btrfs: qgroup: introduce quota mode

In preparation for introducing simple quotas, change from a binary
setting for quotas to an enum based mode. Initially, the possible modes
are disabled/full. Full quotas is normal btrfs qgroups.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>