btrfs: use btrfs_read_node_slot in qgroup_trace_new_subtree_blocks

We're open-coding btrfs_read_node_slot() here, replace with the helper.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>