btrfs: use after free in btrfs_quota_enable

The issue here is that btrfs_commit_transaction() frees "trans" on both
the error and the success path.  So the problem would be if
btrfs_commit_transaction() succeeds, and then qgroup_rescan_init()
fails.  That means that "ret" is non-zero and "trans" is non-NULL and it
leads to a use after free inside the btrfs_end_transaction() macro.

Fixes: 340f1aa ("btrfs: qgroups: Move transaction management inside btrfs_quota_enable/disable")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -1019,10 +1019,9 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
 	spin_unlock(&fs_info->qgroup_lock);
 
 	ret = btrfs_commit_transaction(trans);
-	if (ret) {
-		trans = NULL;
+	trans = NULL;
+	if (ret)
 		goto out_free_path;
-	}
 
 	ret = qgroup_rescan_init(fs_info, 0, 1);
 	if (!ret) {