powerpc/pmap: Fix pte_find_next() iterators for booke64 pmap

After r361988 fixed the reference count leak on booke64, it became possible
for an iteration somewhere in the middle of a page to become stale, with the
page vanishing (correctly) due to all PTEs on that page going away.
pte_find_next() would start at that iterator, and move along 'higher' order
directory pages until it finds a valid one, without zeroing out the lower
order pages. For instance:

/* Find next pte at or above 0x10002000. */
pte = pte_find_next(pmap, &(0x10002000));
pte_remove(pmap, pte);
/* This pte was the last reference in the page table page, page is

• gone.

	 */

pte = pte_find_next(pmap, 0x10002000);
/* pte_find_next will see 0x10002000's page is gone, and jump to the

• next one, but starting iteration at the '0x2000' slot, skipping
• 0x0000 and 0x1000.

	 */

This caused some processes, like git, to trip the KASSERT() in
pmap_release().

Fix this by zeroing all lower order iterators at each level.