BUG/MINOR: debug: only dump the trace once in __BUG_ON_ONCE()

BUG/MINOR: debug: only dump the trace once in __BUG_ON_ONCE()

Amaury found that CHECK_IF() was dumping the stack trace on each call,
which was not the intent. The reason is that complain() performs the
check on the counter, but the decision to dump the stack later is based
on the choice between aborting and dumping. Let's break out of this when
the count is > 1. Under high concurrency it can cause a trace never to
be emitted but this is a detail. The whole thing needs to be redone
more cleanly anyway.

This should be backported to 2.6.