The expression rb_entry(node, struct pstree_item, pid.node)
may create a fake impression, that we dereferrence pstree_item
for threads too, which is a BUG, but it's not so, because
we are only interested in its ->pid field.

But anyway, escape of pstree_item, iterate over struct pid,
which are more readable.

travis-ci: success for Make pstree_item::pid allocated dynamically
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>