btrfs: qgroup: Search commit root for rescan to avoid missing extent

When doing qgroup rescan using the following script (modified from
btrfs/017 test case), we can sometimes hit qgroup corruption.

------
umount $dev &> /dev/null
umount $mnt &> /dev/null

mkfs.btrfs -f -n 64k $dev
mount $dev $mnt

extent_size=8192

xfs_io -f -d -c "pwrite 0 $extent_size" $mnt/foo > /dev/null
btrfs subvolume snapshot $mnt $mnt/snap

xfs_io -f -c "reflink $mnt/foo" $mnt/foo-reflink > /dev/null
xfs_io -f -c "reflink $mnt/foo" $mnt/snap/foo-reflink > /dev/null
xfs_io -f -c "reflink $mnt/foo" $mnt/snap/foo-reflink2 > /dev/unll
btrfs quota enable $mnt

# -W is the new option to only wait rescan while not starting new one
btrfs quota rescan -W $mnt
btrfs qgroup show -prce $mnt
umount $mnt

# Need to patch btrfs-progs to report qgroup mismatch as error
btrfs check $dev || _fail
------

For fast machine, we can hit some corruption which missed accounting
tree blocks:
------
qgroupid rfer excl max_rfer max_excl parent child
-------- ---- ---- -------- -------- ------ -----
0/5 8.00KiB 0.00B none none --- ---
0/257 8.00KiB 0.00B none none --- ---
------

This is due to the fact that we're always searching commit root for
btrfs_find_all_roots() at qgroup_rescan_leaf(), but the leaf we get is
from current transaction, not commit root.

And if our tree blocks get modified in current transaction, we won't
find any owner in commit root, thus causing the corruption.

Fix it by searching commit root for extent tree for
qgroup_rescan_leaf().

Reported-by: Nikolay Borisov <[e-mail]>
Signed-off-by: Qu Wenruo <[e-mail]>
Reviewed-by: Nikolay Borisov <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/qgroup.c

···
 	struct btrfs_key found;
 	struct extent_buffer *scratch_leaf = NULL;
 	struct ulist *roots = NULL;
-	struct seq_list tree_mod_seq_elem = SEQ_LIST_INIT(tree_mod_seq_elem);
 	u64 num_bytes;
 	int slot;
 	int ret;
···
 			      btrfs_header_nritems(path->nodes[0]) - 1);
 	fs_info->qgroup_rescan_progress.objectid = found.objectid + 1;
 
-	btrfs_get_tree_mod_seq(fs_info, &tree_mod_seq_elem);
 	scratch_leaf = btrfs_clone_extent_buffer(path->nodes[0]);
 	if (!scratch_leaf) {
 		ret = -ENOMEM;
···
 		btrfs_tree_read_unlock_blocking(scratch_leaf);
 		free_extent_buffer(scratch_leaf);
 	}
-	btrfs_put_tree_mod_seq(fs_info, &tree_mod_seq_elem);
 
 	return ret;
 }
···
 	path = btrfs_alloc_path();
 	if (!path)
 		goto out;
+	/*
+	 * Rescan should only search for commit root, and any later difference
+	 * should be recorded by qgroup
+	 */
+	path->search_commit_root = 1;
+	path->skip_locking = 1;
 
 	err = 0;
 	while (!err && !btrfs_fs_closing(fs_info)) {