mm, page_alloc: defer debugging checks of freed pages until a PCP drain

mm, page_alloc: defer debugging checks of freed pages until a PCP drain

Every page free checks a number of page fields for validity.  This
catches premature frees and corruptions but it is also expensive.  This
patch weakens the debugging check by checking PCP pages at the time they
are drained from the PCP list.  This will trigger the bug but the site
that freed the corrupt page will be lost.  To get the full context, a
kernel rebuild with DEBUG_VM is necessary.

[[email protected]: fix build]
Signed-off-by: Mel Gorman <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Jesper Dangaard Brouer <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>