netfilter: ctnetlink: Expectations must have a conntrack helper area

The expect check function __nf_ct_expect_check() asks the master_help is
necessary. So it is unnecessary to go ahead in ctnetlink_alloc_expect
when there is no help.

Actually the commit bc01befd ("netfilter: ctnetlink: add support for
user-space expectation helpers") permits ctnetlink create one expect
even though there is no master help. But the latter commit 3d058d7b
("netfilter: rework user-space expectation helper support") disables it
again.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>