tevent: Fix a race condition in tevent context rundown

tevent: Fix a race condition in tevent context rundown

We protect setting tctx->event_ctx=NULL with tctx->event_ctx_mutex.
But in _tevent_threaded_schedule_immediate we have the classic
TOCTOU race: After we checked "ev==NULL", looking at
tevent_common_context_destructor the event context can go after
_tevent_threaded_schedule_immediate checked. We need to serialize
things a bit by keeping tctx->event_ctx_mutex locked while we
reference "ev", in particular in the

DLIST_ADD_END(ev->scheduled_immediates,im);

I think the locking hierarchy is still maintained, tevent_atfork_prepare()
first locks all the tctx locks, and then the scheduled_mutex.  Also,
I don't think this will impact parallelism too badly: event_ctx_mutex
is only used to protect setting tctx->ev.

Found by staring at code while fixing the FreeBSD memleak due to
not destroying scheduled_mutex.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Fri Jun  9 00:45:26 CEST 2017 on sn-devel-144