vm_page: remove pages with iterators

Use pctrie iterators for removing some page sequences from radix
trees, to avoid repeated searches from the tree root.

Rename vm_page_object_remove to vm_page_remove_radixdone, and remove
from it the responsibility for removing a page from its radix tree,
and pass that responsibility on to its callers.

For one of those callers, vm_page_rename, pass a pages pctrie_iter,
rather than a page, and use the iterator to remove the page from its
radix tree.

Define functions vm_page_iter_remove() and vm_page_iter_free() that
are like vm_page_remove() and vm_page_free(), respectively, except
that they take an iterator as parameter rather than a page, and use
the iterator to remove the page from the radix tree instead of
searching the radix tree. Function vm_page_iter_free() assumes that
the page is associated with an object, and calls
vm_page_free_object_prep to do the part of vm_page_free_prep that is
object-related.

In functions vm_object_split and vm_object_collapse_scan, use a
pctrie_iter to walk over the pages of the object, and use
vm_page_rename and vm_radix_iter_remove modify the radix tree without
searching for pages. In vm_object_page_remove and _kmem_unback, use a
pctrie_iter and vm_page_iter_free to remove the page from the radix
tree.

Reviewed by: markj (prevoius version)
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D46724