sync: Fix memory corruption in sync_timeline_signal().

The android_fence_release() function checks for active sync points
by calling list_empty() on the list head embedded on the sync
point. However, it is only valid to use list_empty() on nodes that
have been initialized with INIT_LIST_HEAD() or list_del_init().

Because the list entry has likely been removed from the active list
by sync_timeline_signal(), there is a good chance that this
WARN_ON_ONCE() will be hit due to dangling pointers pointing at
freed memory (even though the sync drivers did nothing wrong)
and memory corruption will ensue as the list entry is removed for
a second time, corrupting the active list.

This problem can be reproduced quite easily with CONFIG_DEBUG_LIST=y
and fences with more than one sync point.

Change-Id: Ie2a6bc1480bbcfdc14f9b385fca5a2b833effc05

Signed-off-by: Alistair Strachan <alistair.strachan@imgtec.com>
Cc: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>