ipvs: Use kthread_run() instead of doing a double-fork via kernel_thread()

This also moves the setup code out of the daemons, so that we're able to
return proper error codes to user space. The current code will return success
to user space when the daemon is started with an invald mcast interface. With
these changes we get an appropriate "No such device" error.

We longer need our own completion to be sure the daemons are actually running,
because they no longer contain code that can fail and kthread_run() takes care
of the rest.

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