btrfs: qgroup: set a more sane default value for subtree drop threshold

Since commit 011b46c30476 ("btrfs: skip subtree scan if it's too high to
avoid low stall in btrfs_commit_transaction()"), btrfs qgroup can
automatically skip large subtree scan at the cost of marking qgroup
inconsistent.

It's designed to address the final performance problem of snapshot drop
with qgroup enabled, but to be safe the default value is
BTRFS_MAX_LEVEL, requiring a user space daemon to set a different value
to make it work.

I'd say it's not a good idea to rely on user space tool to set this
default value, especially when some operations (snapshot dropping) can
be triggered immediately after mount, leaving a very small window to
that that sysfs interface.

So instead of disabling this new feature by default, enable it with a
low threshold (3), so that large subvolume tree drop at mount time won't
cause huge qgroup workload.

CC: [e-mail] # 6.1
Signed-off-by: Qu Wenruo <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/disk-io.c

···
 	fs_info->qgroup_seq = 1;
 	fs_info->qgroup_ulist = NULL;
 	fs_info->qgroup_rescan_running = false;
-	fs_info->qgroup_drop_subtree_thres = BTRFS_MAX_LEVEL;
+	fs_info->qgroup_drop_subtree_thres = BTRFS_QGROUP_DROP_SUBTREE_THRES_DEFAULT;
 	mutex_init(&fs_info->qgroup_rescan_lock);
 }
 

fs/btrfs/qgroup.c

···
 	fs_info->quota_root = NULL;
 	fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_ON;
 	fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_SIMPLE_MODE;
-	fs_info->qgroup_drop_subtree_thres = BTRFS_MAX_LEVEL;
+	fs_info->qgroup_drop_subtree_thres = BTRFS_QGROUP_DROP_SUBTREE_THRES_DEFAULT;
 	spin_unlock(&fs_info->qgroup_lock);
 
 	btrfs_free_qgroup_config(fs_info);

fs/btrfs/qgroup.h

···
 #define BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN		(1ULL << 63)
 #define BTRFS_QGROUP_RUNTIME_FLAG_NO_ACCOUNTING		(1ULL << 62)
 
+#define BTRFS_QGROUP_DROP_SUBTREE_THRES_DEFAULT		(3)
+
 /*
  * Record a dirty extent, and info qgroup to update quota on it
  */