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.