drm/msm/gem: Add missing rcu_dereference()

Fixes a sparse "different address spaces" error.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406280050.syeEwLTE-lkp@intel.com/Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/601612/

drivers/gpu/drm/msm/msm_gem.c

@@ -48,7 +48,7 @@ static void update_ctx_mem(struct drm_file *file, ssize_t size)
 	uint64_t ctx_mem = atomic64_add_return(size, &ctx->ctx_mem);
 
 	rcu_read_lock(); /* Locks file->pid! */
-	trace_gpu_mem_total(0, pid_nr(file->pid), ctx_mem);
+	trace_gpu_mem_total(0, pid_nr(rcu_dereference(file->pid)), ctx_mem);
 	rcu_read_unlock();
 
 }