usercopy: Remove folio references from check_heap_object()

usercopy: Remove folio references from check_heap_object()

Use page_slab() instead of virt_to_folio() followed by folio_slab().
We do end up calling compound_head() twice for non-slab copies, but that
will not be a problem once we allocate memdescs separately.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Cc: linux-hardening@vger.kernel.org
Link: https://patch.msgid.link/20251113000932.1589073-14-willy@infradead.org
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>