mm/slab.c: fix spelling mistake "disired" -> "desired"

 * Because if it is the case, that means we defer the creation of

 * the kmalloc_{dma,}_cache of size sizeof(slab descriptor) to this point.

 * And we eventually call down to __kmem_cache_create(), which

 * in turn looks up in the kmalloc_{dma,}_caches for the disired-size one.

 * in turn looks up in the kmalloc_{dma,}_caches for the desired-size one.

 * This is a "chicken-and-egg" problem.

 *

 * So the off-slab slab descriptor shall come from the kmalloc_{dma,}_caches,