Switch GEOM_DEV from make_dev_p() to make_dev_s().

It closes the race condition and so allows to remove few NULL checks.

Also while there, use dev->si_drv1 in addition to cp->private to store
softc pointer. For calls coming from the dev side it gives reliable cache
hit instead of often miss before.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.