ipvs: null check of net->ipvs in lblc(r) shedulers

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

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