netfilter: ipset: Limit max timeout value

Due to the negative value condition in msecs_to_jiffies(), the real
max possible timeout value must be set to (UINT_MAX >> 1)/MSEC_PER_SEC.

Neutron Soutmun proposed the proper fix, but an insufficient one was
applied, see https://patchwork.ozlabs.org/patch/400405/.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>