device_pager: user iterators to free device pages

Change cdev_mgtdev_page_free_page to take an iterator, rather than an
object and page, so that removing the page from the object radix tree
can take advantage of locality with iterators. Define a
general-purpose function to free all pages, which can be used in
several places.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D47692