drm/i915: Add __rcu annotation to cursor when iterating client objects

__rcu annotation is needed to avoid the sparse warnings such as:

  .../i915_drm_client.c:92:9: sparse: sparse: incompatible types in comparison expression (different address spaces):
  .../i915_drm_client.c:92:9: sparse:    struct list_head [noderef] __rcu *
  .../i915_drm_client.c:92:9: sparse:    struct list_head *

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 9688530 ("drm/i915: Implement fdinfo memory stats printing")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311110610.h0m6ydI5-lkp@intel.com/
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231113085457.199053-2-tvrtko.ursulin@linux.intel.com

Author: tursulin

drivers/gpu/drm/i915/i915_drm_client.c

@@ -78,7 +78,7 @@ static void show_meminfo(struct drm_printer *p, struct drm_file *file)
 	struct drm_i915_private *i915 = fpriv->i915;
 	struct drm_i915_gem_object *obj;
 	struct intel_memory_region *mr;
-	struct list_head *pos;
+	struct list_head __rcu *pos;
 	unsigned int id;
 
 	/* Public objects. */