inet: Avoid ehash lookup race in inet_ehash_insert()

inet: Avoid ehash lookup race in inet_ehash_insert()

Since ehash lookups are lockless, if one CPU performs a lookup while
another concurrently deletes and inserts (removing reqsk and inserting sk),
the lookup may fail to find the socket, an RST may be sent.

The call trace map is drawn as follows:
   CPU 0                           CPU 1
   -----                           -----
				inet_ehash_insert()
                                spin_lock()
                                sk_nulls_del_node_init_rcu(osk)
__inet_lookup_established()
	(lookup failed)
                                __sk_nulls_add_node_rcu(sk, list)
                                spin_unlock()

As both deletion and insertion operate on the same ehash chain, this patch
introduces a new sk_nulls_replace_node_init_rcu() helper functions to
implement atomic replacement.

Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions")
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20251015020236.431822-3-xuanqiang.luo@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>