This patch gets md_open to use mddev_find instead of kdev_to_mddev, thus
creating the mddev if necessary.
This guarantees that md_release will be able to find an mddev to
mddev_put.

Now that we are certain of getting the refcount right at open/close time,
we don't need the "countdev" stuff.  If START_ARRAY happens to start and
array other than that the one that is currently opened, it won't confuse
things at all.