mm: remove page->order

We already use page->private for storing the order of a page while it's in
the buddy allocator system; extend that to also storing the order while
it's in the pcp_llist.

Link: https://lkml.kernel.org/r/20250910142923.2465470-4-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>