mm/swap_state.c: use the same way to count page in [add_to|delete_from]_swap_cache

add_to_swap_cache() and delete_from_swap_cache() are counterparts, while
currently they use different ways to count pages.

It doesn't break anything because we only have two sizes for PageAnon, but
this is confusing and not good practice.

This patch corrects it by making both functions use hpage_nr_pages().
Signed-off-by: NWei Yang <richard.weiyang@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Reviewed-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Link: http://lkml.kernel.org/r/20200315012920.2687-1-richard.weiyang@gmail.comSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/swap_state.c

@@ -116,7 +116,7 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp)
 	struct address_space *address_space = swap_address_space(entry);
 	pgoff_t idx = swp_offset(entry);
 	XA_STATE_ORDER(xas, &address_space->i_pages, idx, compound_order(page));
-	unsigned long i, nr = compound_nr(page);
+	unsigned long i, nr = hpage_nr_pages(page);
 
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(PageSwapCache(page), page);