netfilter: fix checking method of conntrack helper

netfilter: fix checking method of conntrack helper

This patch uses nfct_help() to detect whether an established connection
needs conntrack helper instead of using test_bit(IPS_HELPER_BIT,
&ct->status).

The reason for this modification is that IPS_HELPER_BIT is only set when
the conntrack helper is attached by explicit CT target.

However, in the case that a device enables conntrack helper via the other
ways (e.g., command "echo 1 > /proc/sys/net/netfilter/nf_conntrack_helper")
, the status of IPS_HELPER_BIT will not present any change. That means the
IPS_HELPER_BIT might lose the checking ability in the context.

Signed-off-by: HsiuWen Yen <y.hsiuwen@gmail.com>