BUG/MEDIUM: stick-tables: close a tiny race in __stksess_kill()

BUG/MEDIUM: stick-tables: close a tiny race in __stksess_kill()

It might be possible not to see the element in the tree, then not to
see it in the update list, thus not to take the lock before deleting.
But an element in the list could have moved to the tree during the
check, and be removed later without the updt_lock.

Let's delete prior to checking the presence in the tree to avoid
this situation. No backport is needed since this arrived in -dev13
with the update list.