mm: slab: fix comment for ARCH_KMALLOC_MINALIGN

The comment next to the ARCH_KMALLOC_MINALIGN definition says that
ARCH_KMALLOC_MINALIGN can be defined in arch headers. This is incorrect:
it's actually ARCH_DMA_MINALIGN that can be defined there.

Fix the comment.

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/fe1ca7a25a054b61d1038686d07569416e287e7b.1651161548.git.andreyknvl@google.com