BUG/MEDIUM: mt_lists: Avoid el->prev = el->next = el

BUG/MEDIUM: mt_lists: Avoid el->prev = el->next = el

Avoid setting both el->prev and el->next on the same line.
The goal is to set both el->prev and el->next to el, but a naive
compiler, such as when we're using -O0, will set el->next first, then
will set el->prev to the value of el->next, but if we're unlucky,
el->next will have been set to something else by another thread.
So explicitely set both to what we want.

This should be backported up to 2.8.