mm/z3fold.c: add inter-page compaction

For each page scheduled for compaction (e.  g.  by z3fold_free()), try to
apply inter-page compaction before running the traditional/ existing
intra-page compaction.  That means, if the page has only one buddy, we
treat that buddy as a new object that we aim to place into an existing
z3fold page.  If such a page is found, that object is transferred and the
old page is freed completely.  The transferred object is named "foreign"
and treated slightly differently thereafter.

Namely, we increase "foreign handle" counter for the new page.  Pages with
non-zero "foreign handle" count become unmovable.  This patch implements
"foreign handle" detection when a handle is freed to decrement the foreign
handle counter accordingly, so a page may as well become movable again as
the time goes by.

As a result, we almost always have exactly 3 objects per page and
significantly better average compression ratio.

[cai@lca.pw: fix -Wunused-but-set-variable warnings]
  Link: http://lkml.kernel.org/r/1570542062-29144-1-git-send-email-cai@lca.pw
[vitalywool@gmail.com: avoid subtle race when freeing slots]
  Link: http://lkml.kernel.org/r/20191127152118.6314b99074b0626d4c5a8835@gmail.com
[vitalywool@gmail.com: compact objects more accurately]
  Link: http://lkml.kernel.org/r/20191127152216.6ad33745a21ba71c53606acb@gmail.com
[vitalywool@gmail.com: protect handle reads]
  Link: http://lkml.kernel.org/r/20191127152345.8059852f60947686674d726d@gmail.com
Link: http://lkml.kernel.org/r/20191006041457.24113-1-vitalywool@gmail.comSigned-off-by: Vitaly Wool <vitaly.vul@sony.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Henry Burns <henrywolfeburns@gmail.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>