tools/testing/selftests/mm: add MADV_COLLAPSE test case

tools/testing/selftests/mm: add MADV_COLLAPSE test case

To ensure the retract_page_tables() logic functions correctly with the
introduction of VM_MAYBE_GUARD, add a test to assert that madvise collapse
fails when guard regions are established in the collapsed range in all
cases.

Unfortunately we cannot differentiate between e.g.
CONFIG_READ_ONLY_THP_FOR_FS not being set vs.  a file-backed VMA having
collapse correctly disallowed, so in each instance we will get an assert
pass here.

We add an additional check to see whether guard regions are preserved
across collapse in case of a bug causing the collapse to succeed, which
will give us more data to debug with should this occur in future.

Link: https://lkml.kernel.org/r/0748beeb864525b8ddfa51adad7128dd32eb3ac4.1763460113.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>