rculist: Add hlist_nulls_replace_rcu() and hlist_nulls_replace_init_rcu()

rculist: Add hlist_nulls_replace_rcu() and hlist_nulls_replace_init_rcu()

Add two functions to atomically replace RCU-protected hlist_nulls entries.

Keep using WRITE_ONCE() to assign values to ->next and ->pprev, as
mentioned in the patch below:
commit efd04f8a8b45 ("rcu: Use WRITE_ONCE() for assignments to ->next for
rculist_nulls")
commit 860c8802ace1 ("rcu: Use WRITE_ONCE() for assignments to ->pprev for
hlist_nulls")

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
Link: https://patch.msgid.link/20251015020236.431822-2-xuanqiang.luo@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>