>From the perspective of users, qgroup's referenced or exclusive
is negative,but user can not continue to write data! a workaround
way is to cast u64 to s64 when doing qgroup reservation.
Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com> Reviewed-by: Arne Jansen <sensille@gmx.net> Signed-off-by: Josef Bacik <jbacik@fusionio.com>