netfilter: synproxy: ensure zero is returned on non-error return path

Currently functions nf_synproxy_{ipc4|ipv6}_init return an uninitialized
garbage value in variable ret on a successful return.  Fix this by
returning zero on success.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: d7f9b2f1 ("netfilter: synproxy: extract SYNPROXY infrastructure from {ipt, ip6t}_SYNPROXY")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>