MFC r298072

Don't corrupt ZFS label's physpath attribute when booting while a disk is
missing

Prior to this change, vdev_geom_open_by_path would call vdev_geom_attach
prior to verifying the device's GUIDs. vdev_geom_attach calls
vdev_geom_attrchange to set the physpath in the vdev object. The result is
that if the disk could not be found, then the labels for other disks in the
same TLD would overwrite the missing disk's physpath with the physpath of
whichever disk currently has the same devname as the missing one used to
have.