[SCSI] osd_uld: Bump MAX_OSD_DEVICES from 64 to 1,048,576

It used to be that minors where 8 bit. But now they
are actually 20 bit. So the fix is simplicity itself.

I've tested with 300 devices and all user-mode utils
work just fine. I have also mechanically added 10,000
to the ida (so devices are /dev/osd10000, /dev/osd10001 ...)
and was able to mkfs an exofs filesystem and access osds
from user-mode.

All the open-osd user-mode code uses the same library
to access devices through their symbolic names in
/dev/osdX so I'd say it's pretty safe. (Well tested)

This patch is very important because some of the systems
that will be deploying the 3.2 pnfs-objects code are larger
than 64 OSDs and will stop to work properly when reaching
that number.

CC: Stable <stable@vger.kernel.org>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>