vm_phys: reduce touching of page->pool fields

Change the usage of the pool field in vm_page structs.

Currently, every page belongs to a pool, and the pool field identifies
that pool, whether the page is allocated or free.

With this change, the pool field of the first page of a free block is
used by the buddy allocator to identify its pool, but the buddy
allocator makes no guarantees about the pool field value for allocated
pages. The buddy allocator requires that a pool parameter be passed as
part of freeing memory. A function that allocates memory may use the
pool field of a page to record what pool to pass as that parameter
when the memory is freed, but might not need to do so for every
allocated page.

Suggested by: alc
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D45409