netfilter: ipset: ipset list may return wrong member count for set with timeout

Simple testcase:

$ ipset create test hash:ip timeout 5
$ ipset add test 1.2.3.4
$ ipset add test 1.2.2.2
$ sleep 5

$ ipset l
Name: test
Type: hash:ip
Revision: 5
Header: family inet hashsize 1024 maxelem 65536 timeout 5
Size in memory: 296
References: 0
Number of entries: 2
Members:

We return "Number of entries: 2" but no members are listed. That is
because mtype_list runs "ip_set_timeout_expired" and does not list the
expired entries, but set->elements is never upated (until mtype_gc
cleans it up later).

Reviewed-by: Joshua Hunt <johunt@akamai.com>
Signed-off-by: Vishwanath Pai <vpai@akamai.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>