btrfs: qgroup: do quick checks if quotas are enabled before starting ioctls

The ioctls that add relations, create qgroups or set limits start/join
transaction. When quotas are not enabled this is not necessary, there
will be errors reported back anyway but this could be also misleading
and we should really report that quotas are not enabled. For that use
-ENOTCONN.

The helper is meant to do a quick check before any other standard ioctl
checks are done. If quota is disabled meanwhile we still rely on proper
locking inside any active operation changing the qgroup structures.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>