MFC r243372:

MFC r243372:

Disallow attaching preloaded memory disks via ioctl.

- The feature is dangerous because the kernel code didn't check
  validity of the memory address provided from user space.
- It seems that mdconfig(8) never really supported attaching preloaded
  memory disks.
- Preloaded memory disks are automatically attached during md(4)
  initialization. Thus there shouldn't be much use for the feature.

PR:		kern/169683

git-svn-id: svn://svn.freebsd.org/base/stable/9@245038 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f