tcp: Save lock_sock() for memcg in inet_csk_accept().

tcp: Save lock_sock() for memcg in inet_csk_accept().

If memcg is enabled, accept() acquires lock_sock() twice for each new
TCP/MPTCP socket in inet_csk_accept() and __inet_accept().

Let's move memcg operations from inet_csk_accept() to __inet_accept().

Note that SCTP somehow allocates a new socket by sk_alloc() in
sk->sk_prot->accept() and clones fields manually, instead of using
sk_clone_lock().

mem_cgroup_sk_alloc() is called for SCTP before __inet_accept(),
so I added the protocol check in __inet_accept(), but this can be
removed once SCTP uses sk_clone_lock().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Link: https://patch.msgid.link/20251014235604.3057003-2-kuniyu@google.com