This fixes a NULL ptr dereference in md_probe() noted by Helge Hafting.

It even gets the things slightly better than they used to be, but
late-boot magic in md.c is certainly a mess.  Oh, well...

It's _ugly_.  md_probe() is misused there big way.  The minimal fix is
to revert the cleanup in md_probe() - replace

	int unit = *part;
with
	int unit = MINOR(dev);


However, that is crap solution.  The problem is that md_probe() is called
directly with bogus arguments - not only part is NULL (which triggers the
oops), but dev (which is supposed to be dev_t value) is actually mdidx(mddev).

Cleaner fix follows, but we really need to get the situation with gendisk
allocations into the sane shape there.  Sigh...