stand: Add disk_fmtdev for dv_fmtdev for all the disk devices

All of the archsw fmtdev functions treat DEVT_DISK as a call to
disk_fmtdev. Set all disks' dv_fmtdev to disk_fmtdev so devformat
will return the same thing.

Sponsored by: Netflix
Reviewed by: tsoome (prior version)
Differential Revision: https://reviews.freebsd.org/D35917