fork: Use RCU_INIT_POINTER() instead of rcu_access_pointer()

Use RCU_INIT_POINTER() instead of rcu_access_pointer() in
copy_sighand().

Suggested-by: Oleg Nesterov <[e-mail]>
Signed-off-by: Madhuparna Bhowmik <[e-mail]>
Acked-by: Oleg Nesterov <[e-mail]>
Acked-by: Christian Brauner <[e-mail]>
[[e-mail]: edit commit message]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

kernel/fork.c

···
 		return 0;
 	}
 	sig = kmem_cache_alloc(sighand_cachep, GFP_KERNEL);
-	rcu_assign_pointer(tsk->sighand, sig);
+	RCU_INIT_POINTER(tsk->sighand, sig);
 	if (!sig)
 		return -ENOMEM;