sctp: fix sctp_setsockopt_autoclose compile warning

Fix the following warning, when building on 64 bits:

net/sctp/socket.c:2091: warning: large integer implicitly
                        truncated to unsigned type

Signed-off-by: Andrei Pelinescu-Onciul <andrei@iptel.org>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>