multipath: reference the udev context when starting event queue

multipath: reference the udev context when starting event queue

The uevent listener is running asynchronously, so it might still
be active and receiving events when the main thread is already
shut down. So it need to take a separate reference to the udev
context to avoid the context becoming invalid while the listener
is running.

Signed-off-by: Hannes Reinecke <hare@suse.de>