TLReplaceDisks: Move assertion checking locks

Commit 1bee66f3 added assertions for ensuring only the necessary locks
are kept while replacing disks. One of them makes sure locks have been
released during the operation. Unfortunately the commit added the check
as part of a “finally” branch, which is also run when an exception is
thrown (in which case the locks may not have been released yet). Errors
could be masked by the assertion error. Moving the check out of the
“finally” branch fixes the issue.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>