ipvs: add check in ftp for initialized core

	Avoid crash when registering ip_vs_ftp after
the IPVS core initialization for netns fails. Do this by
checking for present core (net->ipvs).

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Acked-by: Hans Schillstrom <hans.schillstrom@ericsson.com>
Signed-off-by: Simon Horman <horms@verge.net.au>