slab: Remove folio references from __ksize()

slab: Remove folio references from __ksize()

In the future, we will separate slab, folio and page from each other
and calling virt_to_folio() on an address allocated from slab will
return NULL.  Delay the conversion from struct page to struct slab
until we know we're not dealing with a large kmalloc allocation.
There's a minor win for large kmalloc allocations as we avoid the
compound_head() hidden in virt_to_folio().

This deprecates calling ksize() on memory allocated by alloc_pages().
Today it becomes a warning and support will be removed entirely in
the future.

Introduce large_kmalloc_size() to abstract how we represent the size
of a large kmalloc allocation.  For now, this is the same as
page_size(), but it will change with separately allocated memdescs.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20251113000932.1589073-3-willy@infradead.org
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>