ipvs: Only call init_service, update_service and done_service for schedulers if defined

There are schedulers that only schedule based on data available in the service
or destination structures and they don't need any persistent storage or
initialization routine. These schedulers currently provide dummy functions for
the init_service, update_service and/or done_service functions. For the
init_service and done_service cases we already have code that only calls these
functions, if the scheduler provides them. Do the same for the update_service
case and remove the dummy functions from all schedulers.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Signed-off-by: Simon Horman <horms@verge.net.au>