Btrfs: fix missing check before disabling quota

The original code forget to check whether quota has been disabled firstly,
and it will return 'EINVAL' and return error to users if quota has been
disabled,it will be unfriendly and confusing for users to see that.
So just return directly if quota has been disabled.

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Cc: Arne Jansen <sensille@gmx.net>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>