btrfs: qgroup: Don't trace subtree if we're dropping reloc tree

Reloc tree doesn't contribute to qgroup numbers, as we have accounted
them at balance time (see replace_path()).

Skipping the unneeded subtree tracing should reduce the overhead.

[[Benchmark]]
Hardware:
	VM 4G vRAM, 8 vCPUs,
	disk is using 'unsafe' cache mode,
	backing device is SAMSUNG 850 evo SSD.
	Host has 16G ram.

Mkfs parameter:
	--nodesize 4K (To bump up tree size)

Initial subvolume contents:
	4G data copied from /usr and /lib.
	(With enough regular small files)

Snapshots:
	16 snapshots of the original subvolume.
	each snapshot has 3 random files modified.

balance parameter:
	-m

So the content should be pretty similar to a real world root fs layout.

                     | v4.19-rc1    | w/ patchset    | diff (*)
---------------------------------------------------------------
relocated extents    | 22929        | 22900          | -0.1%
qgroup dirty extents | 227757       | 167139         | -26.6%
time (sys)           | 65.253s      | 50.123s        | -23.2%
time (real)          | 74.032s      | 52.551s        | -29.0%

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -8683,7 +8683,13 @@ static noinline int do_walk_down(struct btrfs_trans_handle *trans,
 			parent = 0;
 		}
 
-		if (need_account) {
+		/*
+		 * Reloc tree doesn't contribute to qgroup numbers, and we have
+		 * already accounted them at merge time (replace_path),
+		 * thus we could skip expensive subtree trace here.
+		 */
+		if (root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID &&
+		    need_account) {
 			ret = btrfs_qgroup_trace_subtree(trans, next,
 							 generation, level - 1);
 			if (ret) {