lib/tests/slub_kunit: add a test case for {kmalloc,kfree}_nolock

lib/tests/slub_kunit: add a test case for {kmalloc,kfree}_nolock

Testing invocation of {kmalloc,kfree}_nolock() during kmalloc() or
kfree() is tricky, and it is even harder to ensure that slowpaths are
properly tested. Lack of such testing has led to late discovery of
the bug fixed by commit a1e244a9f177 ("mm/slab: use prandom if
!allow_spin").

Add a slub_kunit test that allocates and frees objects in a tight loop
while a perf event triggers interrupts (NMI or hardirq depending on
the arch) on the same task, invoking {kmalloc,kfree}_nolock() from the
overflow handler.

Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
Link: https://patch.msgid.link/20260406090907.11710-3-harry@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>