netfilter: ctnetlink: fix incorrect NAT expectation dumping

nf_ct_expect_alloc leaves unset the expectation NAT fields. However,
ctnetlink_exp_dump_expect expects them to be zeroed in case they are
not used, which may not be the case. This results in dumping the NAT
tuple of the expectation when it should not.

Fix it by zeroing the NAT fields of the expectation.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>