sctp: fix integer overflow when setting the autoclose timer

When setting the autoclose timeout in jiffies there is a possible
integer overflow if the value in seconds is very large
(e.g. for 2^22 s with HZ=1024). The problem appears even on
64-bit due to the integer promotion rules. The fix is just a cast
 to unsigned long.

Signed-off-by: Andrei Pelinescu-Onciul <andrei@iptel.org>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>