btrfs: qgroup: fix quota disable during rescan

There's a race condition that leads to a NULL pointer dereference if you
disable quotas while a quota rescan is running.  To fix this, we just need
to wait for the quota rescan worker to actually exit before tearing down
the quota structures.
Signed-off-by: NJustin Maggard <jmaggard@netgear.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/qgroup.c

@@ -993,9 +993,10 @@ int btrfs_quota_disable(struct btrfs_trans_handle *trans,
 	mutex_lock(&fs_info->qgroup_ioctl_lock);
 	if (!fs_info->quota_root)
 		goto out;
-	spin_lock(&fs_info->qgroup_lock);
 	fs_info->quota_enabled = 0;
 	fs_info->pending_quota_state = 0;
+	btrfs_qgroup_wait_for_completion(fs_info);
+	spin_lock(&fs_info->qgroup_lock);
 	quota_root = fs_info->quota_root;
 	fs_info->quota_root = NULL;
 	fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_ON;