netfilter: ctnetlink: fix gcc warning during compilation

This patch fixes a (bogus?) gcc warning during compilation:

net/netfilter/nf_conntrack_netlink.c:1234: warning: 'helpname' may be used uninitialized in this function
net/netfilter/nf_conntrack_netlink.c:991: warning: 'helpname' may be used uninitialized in this function

In fact, helpname is initialized by ctnetlink_parse_help() so
I cannot see a way to use it without being initialized.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>