ipvs: Use config macro IS_ENABLED()

Cleanup patch.

Use the IS_ENABLED macro, instead of having to check
both the build and the module CONFIG_ option.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>