selftests/mm: add kmemleak verbose dedup test

selftests/mm: add kmemleak verbose dedup test

Add a regression test for the per-scan verbose dedup added in the
preceding commit.  The test loads samples/kmemleak's helper module
(CONFIG_SAMPLE_KMEMLEAK=m) to generate orphan allocations, several of
which share an allocation backtrace, runs four kmemleak scans with verbose
printing enabled, then walks dmesg looking for two "unreferenced object"
reports within a single scan that share an identical backtrace - which
would mean dedup failed to collapse them.

The test is intentionally permissive on detection but strict on
regressions:

 - PASS when no duplicates are observed, regardless of whether the
   dedup summary line ("... and N more object(s) with the same
   backtrace") was actually emitted. Per-CPU chunk reuse, slab
   freelist pointers, kernel stack residue and CONFIG_DEBUG_KMEMLEAK_
   AUTO_SCAN can all keep most of the orphans "still referenced" or
   reported across many separate scans, so the dedup path may have
   nothing to fold within one scan. That is not a regression.

 - PASS reports whether dedup actually fired, so a passing run on a
   well-behaved environment is still informative.

 - FAIL when two same-backtrace reports land in a single scan (clear
   dedup regression).

 - FAIL when kmemleak's own per-scan tally counts leaks but the
   verbose path emits zero "unreferenced object" lines - that catches
   a regression in the verbose printer itself, which would otherwise
   pass the duplicate check trivially.

 - SKIP when kmemleak is absent, disabled at runtime, or the helper
   module is not built.

The dmesg parser anchors stack-frame matching to the indentation kmemleak
uses for them (4+ spaces under "kmemleak: ") so unrelated kmemleak
warnings landing between reports do not get lumped into the backtrace key
and mask a duplicate.

Link: https://lore.kernel.org/20260506-kmemleak_dedup-v3-2-2d36aafc34da@debian.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>