cache_grow() will call kmem_freepages() if the call to alloc_slabmgmt()
fails.  But the pages have not been marked PageSlab at this stage, so
kmem_freepages() goes BUG.

It is more symmetrical to mark the pages as PageSlab in kmem_getpages().

The patch also prunes a bunch of incorrect comments.

(PageSlab doesn't actually do anything: its only value is as a debug check.
I think the LKCD patch uses it).