Add a uma cache of free pages in the DEFAULT freepool. This gives us
per-cpu alloc and free of pages. The cache is filled with as few trips
to the phys allocator as possible by the use of a new
vm_phys_alloc_npages() function which allocates as many as N pages.

This code was originally by markj with the import function rewritten by
me.

Reviewed by: markj, kib
Tested by: pho
Sponsored by: Netflix, Dell/EMC Isilon
Differential Revision: https://reviews.freebsd.org/D14905