ip_set: Fix build on kernels without system_power_efficient_wq

ip_set: Fix build on kernels without system_power_efficient_wq

This type of workqueue according to comment in <linux/workqueue.h> is
same as regular @system_wq when power efficiency scheduler options
disabled.

For old kernels where @system_power_efficient_wq is not available use
regular @system_wq to support ipset on kernels below 3.11.

Fixes: 33f08da28324 ("netfilter: ipset: Fix "INFO: rcu detected stall in hash_xxx" reports")
Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>