dma-buf: use inline lock for the stub fence v2

dma-buf: use inline lock for the stub fence v2

Using the inline lock is now the recommended way for dma_fence
implementations.

So use this approach for the framework's internal fences as well.

Also saves about 4 bytes for the external spinlock.

v2: drop unnecessary changes

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Philipp Stanner <phasta@kernel.org>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/r/20260219160822.1529-7-christian.koenig@amd.com