Merge branch 'multi-split-btf-fixes-and-test'

Alan Maguire says:

====================
Multi-split BTF fixes and test

This small series consists of a fix to multi-split BTF parsing
(patch 1) and a test which exercises (multi-)split BTF parsing
(patch 2).

Changes since v3 [1]
- add asserts to ensure number of types in original and parsed
  BTF are identical, and the calls to btf__type_by_id() return
  valid pointers (code review bot, patch 2)

Changes since v2 [2]

- fix Fixes: tag formatting (Andrii, patch 1)
- BPF code-review bot saw we were doing ASSERT_OK_PTR() on wrong
  BTF (not multisplit) in patch 2
- ensure cleanup is correctly handled for BTF, unlink in split
  tests (Andrii, patch 2)

Changes since v1 [3]

- BPF code-review bot spotted another place that the string offset
needed to be adjusted based upon base start string offset + header
string offset.
- added selftests to extend split BTF testing to parsing

[1] https://lore.kernel.org/bpf/20251028225544.1312356-1-alan.maguire@oracle.com/
[2] https://lore.kernel.org/bpf/20251028155709.1265445-1-alan.maguire@oracle.com/
[3] https://lore.kernel.org/bpf/20251023142812.258870-1-alan.maguire@oracle.com/
====================

Link: https://patch.msgid.link/20251104203309.318429-1-alan.maguire@oracle.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>