slub: remove use of page->flags

Use slub->__page_flags instead.  We can also remove the assertion that
it's not a tail page as struct slab never points to a tail page.

Link: https://lkml.kernel.org/r/20240326171045.410737-9-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/slub.c

@@ -616,18 +616,12 @@ slub_set_cpu_partial(struct kmem_cache *s, unsigned int nr_objects)
  */
 static __always_inline void slab_lock(struct slab *slab)
 {
-	struct page *page = slab_page(slab);
-
-	VM_BUG_ON_PAGE(PageTail(page), page);
-	bit_spin_lock(PG_locked, &page->flags);
+	bit_spin_lock(PG_locked, &slab->__page_flags);
 }
 
 static __always_inline void slab_unlock(struct slab *slab)
 {
-	struct page *page = slab_page(slab);
-
-	VM_BUG_ON_PAGE(PageTail(page), page);
-	bit_spin_unlock(PG_locked, &page->flags);
+	bit_spin_unlock(PG_locked, &slab->__page_flags);
 }
 
 static inline bool