Replace redundant code with a few new vm_page_grab facilities:

• VM_ALLOC_NOCREAT will grab without creating a page.
• vm_page_grab_valid() will grab and page in if necessary.
• vm_page_busy_acquire() automates some busy acquire loops.

Discussed with: alc, kib, markj
Tested by: pho (part of larger branch)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21546