ipvs: Pass ipvs into ensure_mtu_is adequate

This allows two different ways for computing/guessing net to be
removed from ensure_mtu_is_adequate.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>