From: NeilBrown <neilb@cse.unsw.edu.au>

The "START_ARRAY" ioctl depends on major/minor numbers (as stored in the raid
superblock) are stable over reboots, which is increasingly untrue.

There are better ways to start an array (e.g.  with mdadm) so we mark the
ioctl as deprecated for 2.6, and will remove it in 2.7.