Rationale:  devfs_only does nothing but disabling {un,}register_blkdev
and {un,}register_chrdev.  {un,}register_blkdev already do nothing but
adding it's name argument to a lookup table for the __bdevname and
/proc/device output so this use is already bogus.  The disabling of
the character device per-major arrays can work in practice but is
useless as any driver relying on it can't be used on non-devfs systems.