It turns out that it is only safe to call blkdev_ioctl when the device
is actually open (as ->bd_disk is set to NULL on last close).  And it
is quite possible for do_md_stop to be called when the device is not
open.  So discard the call to blkdev_ioctl(BLKRRPART) which was
added in
   commit 934d9c23

It is just as easy to call this ioctl from userspace when needed (on
mdadm -S) so leave it out of the kernel
Signed-off-by: NeilBrown <neilb@suse.de>