vm_page_grab_pages: avoid doomed range lookups

In vm_page_grab_pages, if a range of grabbed pages has just been
exhausted, then an immediate request for a new range is futile; we
know there's no page there. So only try to lookup more ranges after
allocating a page.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D50394