Move checks for provider's sectorsize and mediasize into a single
location in vdev_geom_attach. Remove the zfs::vdev::taste class;
it's ok to use the regular vdev class for tasting. Consolidate guid
checks into a single location in vdev_attach_ok. Consolidate some
error handling code from vdev_geom_attach into vdev_geom_detach,
closing a resource leak of geom consumers in the process.