mm: slab: fix comment for __assume_kmalloc_alignment

The comment next to the __assume_kmalloc_alignment definition is not
precise: kmalloc relies on kmem_cache_alloc, so kmalloc technically returns
pointers aligned to both ARCH_KMALLOC_MINALIGN and ARCH_SLAB_MINALIGN,
not only to ARCH_KMALLOC_MINALIGN.

(See create_kmalloc_cache()->create_boot_cache()->calculate_alignment()
 for SLAB and SLUB and __do_kmalloc_node() for SLOB.)

Clarify the comment.

The assumption specified by __assume_kmalloc_alignment is still correct,
although it can be made stronger. I'll leave this to a separate patch.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/r/84d8142747230f2015eaf9705ee7c2e1a9f56596.1651161548.git.andreyknvl@google.com