Remove the DKF_LABELVALID flag from struct disk. Instead, trust disk drivers

Remove the DKF_LABELVALID flag from struct disk. Instead, trust disk drivers
to always be able to provide a duid, and keep ignoring whole zero duids.

This fixes a race in vnd setup where the disk_attach callback could run
before any I/O occurs on the vnd, thus not having a label available yet.

noticed by otto@ and kn@; ok kn@