mm/z3fold: throw warning on failure of trylock_page in z3fold_alloc

If trylock_page fails, the page won't be non-lru movable page.  When this
page is freed via free_z3fold_page, it will trigger bug on PageMovable
check in __ClearPageMovable.  Throw warning on failure of trylock_page to
guard against such rare case just as what zsmalloc does.

Link: https://lkml.kernel.org/r/20220429064051.61552-5-linmiaohe@huawei.comSigned-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/z3fold.c

@@ -1122,11 +1122,10 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
 		__SetPageMovable(page, pool->inode->i_mapping);
 		unlock_page(page);
 	} else {
-		if (trylock_page(page)) {
+		WARN_ON(!trylock_page(page));
 		__SetPageMovable(page, pool->inode->i_mapping);
 		unlock_page(page);
 	}
-	}
 	z3fold_page_lock(zhdr);
 
 found: