vm_object: use iterators in page_collect_flush()

[dougm]

Summary

Implement vm_object_page_collect_flush() using iterators instead of vm_page_next() and vm_page_prev().

[kib]

sys/vm/vm_object.c

1163

vm_pageout_flush() calls vm_pager_put_pages(), which implementations drop the object lock. Wouldn't this invalidate the iterator in the main loop in vm_object_page_clean(). In particular, the old 'goto rescan' condition is only sometimes to help.

[dougm]

Always reset the iterator after calling vm_object_page_collect_flush.