sctp: fix sockets_allocated imbalance after sk_clone()

sk_clone() increments sockets_allocated and sets the socket refcount to 2.
SCTP performs additional accounting in sctp_clone_sock(), so the clone-time
increment must be undone to avoid double counting.

Note we cannot simply remove the SCTP-side increment, because the SCTP
destroy path in sctp_destroy_sock() only decrements sockets_allocated when
sp->ep is set, which may not be true for all failure paths in
sctp_clone_sock().

Fixes: 16942cf ("sctp: Use sk_clone() in sctp_accept().")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/af8d66f928dec3e9fcbee8d4a85b7d5a6b86f515.1776460180.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: lxin

net/sctp/socket.c

@@ -4855,8 +4855,9 @@ static struct sock *sctp_clone_sock(struct sock *sk,
 	if (!newsk)
 		return ERR_PTR(err);
 
-	/* sk_clone() sets refcnt to 2 */
+	/* sk_clone() sets refcnt to 2 and increments sockets_allocated */
 	sock_put(newsk);
+	sk_sockets_allocated_dec(newsk);
 
 	newinet = inet_sk(newsk);
 	newsp = sctp_sk(newsk);