selftests/nommu: add nommu mmap and mremap behavior tests

Introduce a kselftest utility to validate memory mapping capabilities
under nommu kernels, aligned with
Documentation/admin-guide/mm/nommu-mmap.rst.

The test implements basic checks into a generic architecture-agnostic
test matrix applicable across nommu targets. It evaluates:

1. MAP_FIXED allocation rejections.
2. Standard MAP_PRIVATE and MAP_ANONYMOUS allocation resilience.
3. Regular file mappings via standard filesystem storage.
4. Block device subsystem mappings (gracefully skipping if node is
   missing).
5. Shared vs Private backing discrepancies under nommu conditions.
6. mremap limits, ensuring non-expandable restrictions behave properly.

Link: https://lore.kernel.org/20261006085642.3852152-3-thehajime@gmail.com
Signed-off-by: Hajime Tazaki <thehajime@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260825015945.141739-1-thehajime@gmail.com
Assisted-by: Gemini:Pro [AI_Reviewer] [Sashiko_Linter]
Assisted-by: cubic.dev:unspecified
Cc: David Hildenbrand <david@kernel.org>
Cc: "Liam R. Howlett" <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Ricardo B . Marlière <rbm@suse.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Brendan Jackman <brendan.jackman@linux.dev>
Cc: Hangbin Liu <liuhangbin@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <kees@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Drewry <wad@chromium.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Daniel Palmer <daniel@thingy.jp>

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