selftests/bpf: Add a test for kmem_cache_iter

selftests/bpf: Add a test for kmem_cache_iter

The test traverses all slab caches using the kmem_cache_iter and save
the data into slab_result array map.  And check if current task's
pointer is from "task_struct" slab cache using bpf_get_kmem_cache().

Also compare the result array with /proc/slabinfo if available (when
CONFIG_SLUB_DEBUG is on).  Note that many of the fields in the slabinfo
are transient, so it only compares the name and objsize fields.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20241010232505.1339892-4-namhyung@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>