netfilter: ipset: use setup_timer() and mod_timer().

Use setup_timer() and instead of init_timer(), being the preferred way
of setting up a timer.

Also, quoting the mod_timer() function comment:
-> mod_timer() is a more efficient way to update the expire field of an
   active timer (if the timer is inactive it will be activated).

Use setup_timer() and mod_timer() to setup and arm a timer, making the
code compact and easier to read.

Signed-off-by: Muhammad Falak R Wani <falakreyaz@gmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>