diskutil: ensure that we lock around disk_list access

diskutil: ensure that we lock around disk_list access

And also ensure that before we free the disk util structures,
we wait for the disk util thread to exit.

This is v2 of the patch. Commit feb41855 had a bug where it
would deadlock on the disk_util_mutex, when handling drives
that had slaves (lvm/md).

Signed-off-by: Jens Axboe <axboe@kernel.dk>