Btrfs: don't call btrfs_qgroup_free if just btrfs_qgroup_reserve fails

commit eb6b88d92c6df083dd09a8c471011e3788dfd7c6 leads into another bug.
If it is just because qgroup_reserve fails, the function btrfs_qgroup_free
should not be called, otherwise, it will cause the wrong quota accounting.

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>