Adjust major number testing to allow for extended minor number in 2.6.28

Adjust major number testing to allow for extended minor number in 2.6.28

From 2.6.28, normal md device will be able to have partitions.  These
partitions will have a different major number.  Sometimes mdadm tests
the major number and so can get confused.
Change these tests to test against get_mdp_major().  mdp does not use
extended minor number and so this test will always be accurate.

Also use /sys/dev links to map major/minor to devnum in sysfs.

Signed-off-by: NeilBrown <neilb@suse.de>