netfilter: ipset: Fix static checker warning in ip_set_core.c

Dan Carpenter reported the following static checker warning:

        net/netfilter/ipset/ip_set_core.c:1414 call_ad()
        error: 'nlh->nlmsg_len' from user is not capped properly

The payload size is limited now by the max size of size_t.

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