sctp: Prevent TOCTOU out-of-bounds write

For the following path not holding the sock lock,

sctp_diag_dump() -> sctp_for_each_endpoint() -> sctp_ep_dump()

make sure not to exceed bounds in case the address list has grown
between buffer allocation (time-of-check) and write (time-of-use).

Suggested-by: Kuniyuki Iwashima <[e-mail]>
Fixes: 8f840e47f190 ("sctp: add the sctp_diag.c file")
Signed-off-by: Stefan Wiehler <[e-mail]>
Reviewed-by: Kuniyuki Iwashima <[e-mail]>
Acked-by: Xin Long <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/sctp/diag.c

···
 		memcpy(info, &laddr->a, sizeof(laddr->a));
 		memset(info + sizeof(laddr->a), 0, addrlen - sizeof(laddr->a));
 		info += addrlen;
+
+		if (!--addrcnt)
+			break;
 	}
 	rcu_read_unlock();