netfilter: nf_ct_expect: nf_ct_expect_related_report(): Return zero on success.

Commit 4dee62b1 ("netfilter: nf_ct_expect: nf_ct_expect_insert()
returns void") inadvertently changed the successful return value of
nf_ct_expect_related_report() from 0 to 1, which caused openvswitch
conntrack integration fail in FTP test cases.

Fix this by always returning zero on the success code path.

Fixes: 4dee62b1 ("netfilter: nf_ct_expect: nf_ct_expect_insert() returns void")
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>