Mutex<Work> → Work (#1156)

I missed this one in #1132, but it's another case where the mutex is
unnecessary.

Exclusive (or shared + immutable) borrowing of the `Work` is guaranteed by the
borrow checker, because the `struct Work` is exclusively owned by the
`Downstairs`.

Therefore, I replaced `Downstairs::work_lock` with `Downstairs::work` and
`Downstairs::work_mut`, retaining the same checks for whether the upstairs is
active.  The relevant portion of new code is
[here](https://github.com/oxidecomputer/crucible/compare/main...mkeeter:more-mutex-removal?expand=1#diff-ccb2591a32a7a66c4c4cbb1125ce4e72d2d1b2f10e805991a30f9e8643e3d954R1945-R1964);
most everything else is boilerplate (since like #1132, removing the async borrow
changes other functions from `async` to sync).