ddf tests: fix get_rootdev

Getting the major number from the hex device number should take
all-but-the-last-two digits, rather than just the first two digits.

Signed-off-by: NeilBrown <neilb@suse.de>
IMSM metadata really should be ignored when found on partitions.

commit b31df43682216d1c65813eae49ebdd8253db8907
changed load_super_imsm to not insist on finding a partition if
ignore_hw_compat was set.
Unfortunately this is set for '--assemble' so arrays could get
assembled badly.

The comment says this was to allow e.g. --examine of image files.
A better fixes for this is to change test_partitions to not report
a regular file as being a partition.
The errors from the BLKPG ioctl are:

 ENOTTY : not a block device.
 EINVAL : not a whole device (probably a partition)
 ENXIO  : partition doesn't exist (so not a partition)

Reported-by: "David F." <df7729@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>