Turns out mscan deals just fine with the benchmark setup

Also, don't clear PTEs that belong to other VMAs when COW faulting.

Fixes sharing on distref's lazy init flag

..running the GC and retrying doesn't usually help free up stacks.
The parent thread needs to wait.

Previously, when we read-faulted on a file-mapped COW page (which can
happen because we don't load anything in to a forked address space's
page tables), we would unconditionally readi to fill that page's
contents, even though that page had often been allocated and filled by
another process (possibly long ago).  This combines allocating and
loading a page into one atomic step that only happens if the page has
not already been loaded into the vmnode.

Coincidentally, this eliminates all unexpected sharing from dirbench.

So scale-amd64 can build lwIP again..

This have to do with implementation, not interface.  Removing these
unmasks the sharing on tlbflush_req, which is something we expected.
Luckily, it doesn't expose any other sharing.