mm/z3fold.c: add structure for buddy handles

mm/z3fold.c: add structure for buddy handles

For z3fold to be able to move its pages per request of the memory
subsystem, it should not use direct object addresses in handles.  Instead,
it will create abstract handles (3 per page) which will contain pointers
to z3fold objects.  Thus, it will be possible to change these pointers
when z3fold page is moved.

Link: http://lkml.kernel.org/r/20190417103826.484eaf18c1294d682769880f@gmail.com
Signed-off-by: Vitaly Wool <vitaly.vul@sony.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Cc: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>