Update sd and ide-disk such that they can take advantage of extended
minors.

ide-disk already has 64 minors per device and currently doesn't use
extended minors although after this patch it can be turned on by
simply tweaking constants.

sd only had 16 minors per device causing problems on certain peculiar
configurations.  This patch lifts the restriction and enables it to
use upto 64 minors.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>