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.