dma-buf/sw-sync: Fix locking around sync_timeline lists

The sync_pt were not adding themselves atomically to the timeline lists,
corruption imminent.  Only a single list is required to track the
unsignaled sync_pt, so reduce it and rename the lock more appropriately
along with using idiomatic names to distinguish a list from links along
it.

v2: Prevent spinlock recursion on free during create (next patch) and
fixup crossref in kerneldoc

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Gustavo Padovan <gustavo@padovan.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170629210532.5617-1-chris@chris-wilson.co.uk