sctp: fix incorrect overflow check on autoclose

Commit 8ffd3208 voids the previous patches f6778aab and 810c0719 for
limiting the autoclose value.  If userspace passes in -1 on 32-bit
platform, the overflow check didn't work and autoclose would be set
to 0xffffffff.

This patch defines a max_autoclose (in seconds) for limiting the value
and exposes it through sysctl, with the following intentions.

1) Avoid overflowing autoclose * HZ.

2) Keep the default autoclose bound consistent across 32- and 64-bit
   platforms (INT_MAX / HZ in this patch).

3) Keep the autoclose value consistent between setsockopt() and
   getsockopt() calls.

Suggested-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sctp/structs.h

@@ -241,6 +241,9 @@ extern struct sctp_globals {
 	 * bits is an indicator of when to send and window update SACK.
 	 */
 	int rwnd_update_shift;
+
+	/* Threshold for autoclose timeout, in seconds. */
+	unsigned long max_autoclose;
 } sctp_globals;
 
 #define sctp_rto_initial		(sctp_globals.rto_initial)
@@ -281,6 +284,7 @@ extern struct sctp_globals {
 #define sctp_auth_enable		(sctp_globals.auth_enable)
 #define sctp_checksum_disable		(sctp_globals.checksum_disable)
 #define sctp_rwnd_upd_shift		(sctp_globals.rwnd_update_shift)
+#define sctp_max_autoclose		(sctp_globals.max_autoclose)
 
 /* SCTP Socket type: UDP or TCP style. */
 typedef enum {