vm_grab_valid: make one with iter param

[dougm]

Summary

Define vm_page_grab_valid_iter() to be the function that does the work of vm_page_grab(), but has an iterator parameter passed to it. In a couple of places, this can be called directly to let an iterator get reused.

[markj]

sys/vm/vm_page.c

5012

Need to reset the iterator here.

[markj]

sys/vm/vm_page.c

5017

Sadly, it's possible for vm_pager_has_page() to drop the object lock, see vnode_pager_haspage() for an example. So the iterator needs to be reinited after this point.

More generally, I think it's ok to assume that vm_pager_has_page() does not drop the object lock if the VM object is swap-backed, and in particular, if it has OBJ_ANON set. While that's not always true here, I think the use in vm_object_collapse_scan() is safe with respect to iterators.

[dougm]

Add necessary iterator resets.