kasan: rename kasan_poison_kfree

Rename kasan_poison_kfree() into kasan_slab_free_mempool() as it better
reflects what this annotation does.

No functional changes.

Co-developed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Link: https://linux-review.googlesource.com/id/I5026f87364e556b506ef1baee725144bb04b8810