ipvs: prevent mixing heterogeneous pools and synchronization

The synchronization protocol is not compatible with heterogeneous pools, so
we need to verify that we're not turning both on at the same time.

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