ipvs: register conntrack hooks for ftp

ip_vs_ftp requires conntrack modules for mangling
of FTP command responses in passive mode.

Make sure the conntrack hooks are registered when
real servers use NAT method in FTP virtual service.
The hooks will be registered while the service is
present.

Fixes: 0c66dc1e ("netfilter: conntrack: register hooks in netns when needed by ruleset")
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>