btrfs: add cond_resched to btrfs_qgroup_trace_leaf_items

On an uncontended system, we can end up hitting soft lockups while
doing replace_path.  At the core, and frequently called is
btrfs_qgroup_trace_leaf_items, so it makes sense to add a cond_resched
there.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>