atomic: fix atomic_add_unless() fallback's return value

According to the kernel documentation:
  Returns non-zero if @v was not @u, and zero otherwise.

Fixes: 63fc5718 ("atomic: add atomic_add_unless()")
Closes: https://gitlab.freedesktop.org/mesa/drm/issues/17


Signed-off-by: David Shao <davshao@gmail.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>

[Eric: fix its callers to maintain current behaviour]
Signed-off-by: Eric Engestrom <eric@engestrom.ch>