Merge branch 'bpf-arena-add-kfunc-for-reserving-arena-memory'

Emil Tsalapatis says:

====================
bpf/arena: Add kfunc for reserving arena memory

Add a new kfunc for BPF arenas that reserves a region of the mapping
to prevent it from being mapped. These regions serve as guards against
out-of-bounds accesses and are useful for debugging arena-related code.

>From v3 (20250709015712.97099-1-emil@etsalapatis.com)
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- Added Acked-by tags by Yonghong.
- Replace hardcoded error numbers in selftests (Yonghong).
- Fixed selftest for partially freeing a reserved region (Yonghong).

>From v2 (20250702003351.197234-1-emil@etsalapatis.com)
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- Removed -EALREADY and replaced with -EINVAL to bring error handling in
  line with the rest of the BPF code (Alexei).

>From v1 (20250620031118.245601-1-emil@etsalapatis.com)
------------------------------------------------------

- Removed the additional guard range tree. Adjusted tests accordingly.
  Reserved regions now behave like allocated regions, and can be
  unreserved using bpf_arena_free_pages(). They can also be allocated
  from userspace through minor faults. It is up to the user to prevent
  erroneous frees and/or use the BPF_F_SEGV_ON_FAULT flag to catch
  stray userspace accesses (Alexei).
- Changed terminology from guard pages to reserved pages (Alexei,
  Kartikeya).

Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
====================

Link: https://patch.msgid.link/20250709191312.29840-1-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>