net/sctp: Validate parameter size for SCTP_GET_ASSOC_STATS

Building sctp may fail with:

In function ‘copy_from_user’,
    inlined from ‘sctp_getsockopt_assoc_stats’ at
    net/sctp/socket.c:5656:20:
arch/x86/include/asm/uaccess_32.h:211:26: error: call to
    ‘copy_from_user_overflow’ declared with attribute error: copy_from_user()
    buffer size is not provably correct

if built with W=1 due to a missing parameter size validation
before the call to copy_from_user.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
static void __sctp_write_space(struct sctp_association *asoc)
{
	struct sock *sk = asoc->base.sk;
	struct socket *sock = sk->sk_socket;

	if ((sctp_wspace(asoc) > 0) && sock) {
		if (waitqueue_active(&asoc->wait))
			wake_up_interruptible(&asoc->wait);

		if (sctp_writeable(sk)) {
			wait_queue_head_t *wq = sk_sleep(sk);

			if (wq && waitqueue_active(wq))
				wake_up_interruptible(wq);

			/* Note that we try to include the Async I/O support
			 * here by modeling from the current TCP/UDP code.
			 * We have not tested with it yet.
			 */
			if (!(sk->sk_shutdown & SEND_SHUTDOWN))
				sock_wake_async(sock,
						SOCK_WAKE_SPACE, POLL_OUT);
		}
	}
}

static void __sctp_write_space(struct sctp_association *asoc)
{
	struct sock *sk = asoc->base.sk;
	struct socket *sock = sk->sk_socket;

	if ((sctp_wspace(asoc) > 0) && sock) {
		if (waitqueue_active(&asoc->wait))
			wake_up_interruptible(&asoc->wait);

		if (sctp_writeable(sk)) {
			wait_queue_head_t *wq = sk_sleep(sk);

			if (wq && waitqueue_active(wq))
				wake_up_interruptible(wq);

			/* Note that we try to include the Async I/O support
			 * here by modeling from the current TCP/UDP code.
			 * We have not tested with it yet.
			 */
			if (!(sk->sk_shutdown & SEND_SHUTDOWN))
				sock_wake_async(sock,
						SOCK_WAKE_SPACE, POLL_OUT);
		}
	}
}
