vm_object: iterate to add pages in reconstruction

[dougm]

Summary

Where vm_page_rename(), vm_object_collapse_scan(), and
vm_object_split() move pages from a source object to a destination,
use an iterator to add to the destination, as we already use an
iterator to remove from the source.

Also, remove a leftover, purposeless assignment.

[dougm]

Drop the fix that @kib has committed separately.

[markj]

sys/vm/vm_page.c

2108

Shouldn't this result in a panic? A caller renaming a page over an existing page in the new object would be a bug.

sys/vm/vm_page.h

687–688

The parameter names don't match those in the function definition.

[dougm]

Add a panic when a page duplication is found. Match param names between definition and declaration.

Rebase, assuming this will be applied after D47829.

[alc]

This change increased the cycles spent in both vm_object_split() and vm_object_collapse_scan() during a buildworld. It appears that we're not moving enough pages to overcome the startup costs of the iterator.

[dougm]

Rebase. Testing shows this degrades performance, still, so dropping reviewers.