This will allow us to know, in the event of a device failure, when the
device is completely unused and so can be disconnected from the
array.  Currently this isn't a problem as drives aren't normally disconnect
until after a repacement has been rebuilt, which is a LONG TIME, but that
will change shortly...

We always increment the count under a spinlock after checking that
it hasn't been disconnected already (rdev!= NULL).
We disconnect under the same spinlock after checking that the
count is zero.