sctp: Don't call sk->sk_prot->init() in sctp_v[46]_create_accept_sk().

sctp: Don't call sk->sk_prot->init() in sctp_v[46]_create_accept_sk().

sctp_accept() calls sctp_v[46]_create_accept_sk() to allocate a new
socket and calls sctp_sock_migrate() to copy fields from the parent
socket to the new socket.

sctp_v[46]_create_accept_sk() calls sctp_init_sock() to initialise
sctp_sock, but most fields are overwritten by sctp_copy_descendant()
called from sctp_sock_migrate().

Things done in sctp_init_sock() but not in sctp_sock_migrate() are
the following:

  1. Copy sk->sk_gso
  2. Copy sk->sk_destruct (sctp_v6_init_sock())
  3. Allocate sctp_sock.ep
  4. Initialise sctp_sock.pd_lobby
  5. Count sk_sockets_allocated_inc(), sock_prot_inuse_add(),
     and SCTP_DBG_OBJCNT_INC()

Let's do these in sctp_copy_sock() and sctp_sock_migrate() and avoid
calling sk->sk_prot->init() in sctp_v[46]_create_accept_sk().

Note that sk->sk_destruct is already copied in sctp_copy_sock().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20251023231751.4168390-4-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>