netfilter: ipset: Fix set:list type crash when flush/dump set in parallel

Flushing/listing entries was not RCU safe, so parallel flush/dump
could lead to kernel crash. Bug reported by Deniz Eren.

Fixes netfilter bugzilla id #1050.

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