First step to tidying mddev recounting and locking.

This patches introduces
  mddev_get   which incs the refcount on an mddev
  mddev_put   which decs it and, if it becomes unused, frees it
  mddev_find  which finds or allocated an mddev for a given minor
              This is mostly the old alloc_mddev


free_mddev no longer actually frees it.  It just disconnects all drives
so that mddev_put will do the free.

Now the test for "does an mddev exist" is not "mddev != NULL"
but involves checking if the mddev has disks or a superblock
attached.

This makes the semantics of do_md_stop a bit cleaner.  Previously
if do_md_stop succeed for a real stop (not a read-only stop) then
you didn't have to unlock the mddev, otherwise you did.  Now
you always unlock the mddev after do_md_stop.