IPVS: ip_vs_{un,}bind_scheduler NULL arguments

In general NULL arguments aren't passed by the few callers that exist,
so don't test for them.

The exception is to make passing NULL to ip_vs_unbind_scheduler() a noop.

Signed-off-by: Simon Horman <horms@verge.net.au>
Acked-by: Julian Anastasov <ja@ssi.bg>