mm/page_alloc: move free_the_page

Patch series "Allow high order pages to be stored on PCP", v2.

The per-cpu page allocator (PCP) only handles order-0 pages.  With the
series "Use local_lock for pcp protection and reduce stat overhead" and
"Calculate pcp->high based on zone sizes and active CPUs", it's now
feasible to store high-order pages on PCP lists.

This small series allows PCP to store "cheap" orders where cheap is
determined by PAGE_ALLOC_COSTLY_ORDER and THP-sized allocations.

This patch (of 2):

In the next page, free_compount_page is going to use the common helper
free_the_page.  This patch moves the definition to ease review.  No
functional change.

Link: https://lkml.kernel.org/r/20210603142220.10851-1-mgorman@techsingularity.net
Link: https://lkml.kernel.org/r/20210603142220.10851-2-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>