dma-fence: Fix sparse warnings due __rcu annotations

dma-fence: Fix sparse warnings due __rcu annotations

__rcu annotations on the return types from dma_fence_driver_name() and
dma_fence_timeline_name() cause sparse to complain because both the
constant signaled strings, and the strings return by the dma_fence_ops are
not __rcu annotated.

For a simple fix it is easiest to cast them with __rcu added and undo the
smarts from the tracpoints side of things. There is no functional change
since the rest is left in place. Later we can consider changing the
dma_fence_ops return types too, and handle all the individual drivers
which define them.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Fixes: 506aa8b02a8d ("dma-fence: Add safe access helpers and document the rules")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506162214.1eA69hLe-lkp@intel.com/
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20250616155952.24259-1-tvrtko.ursulin@igalia.com
Signed-off-by: Christian König <christian.koenig@amd.com>