Step to reproduce:
	mkfs.btrfs <disk>
	mount <disk> <mnt>
	dd if=/dev/zero of=/<mnt>/data bs=1M count=10
	sync
	btrfs quota enable <mnt>
	btrfs qgroup create 0/5 <mnt>
	btrfs qgroup limit 5M 0/5 <mnt>
	rm -f /<mnt>/data
	sync
	btrfs qgroup show <mnt>
	dd if=/dev/zero of=data bs=1M count=1

>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>