mm/zbud: don't export any zbud API

The zbud doesn't need to export any API and it is meant to be used via
zpool API since the commit 12d79d64bf ("mm/zpool: update zswap to use
zpool").  So we can remove the unneeded zbud.h and move down zpool API to
avoid any forward declaration.

[linmiaohe@huawei.com: fix unused function warnings when CONFIG_ZPOOL is disabled]
  Link: https://lkml.kernel.org/r/20210619025508.1239386-1-linmiaohe@huawei.com

Link: https://lkml.kernel.org/r/20210608114515.206992-3-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

MAINTAINERS

@@ -20172,7 +20172,6 @@ M:	Seth Jennings <sjenning@redhat.com>
 M:	Dan Streetman <ddstreet@ieee.org>
 L:	linux-mm@kvack.org
 S:	Maintained
-F:	include/linux/zbud.h
 F:	mm/zbud.c
 
 ZD1211RW WIRELESS DRIVER

include/linux/zbud.h

@@ -1,23 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0 */
-#ifndef _ZBUD_H_
-#define _ZBUD_H_
-
-#include <linux/types.h>
-
-struct zbud_pool;
-
-struct zbud_ops {
-	int (*evict)(struct zbud_pool *pool, unsigned long handle);
-};
-
-struct zbud_pool *zbud_create_pool(gfp_t gfp, const struct zbud_ops *ops);
-void zbud_destroy_pool(struct zbud_pool *pool);
-int zbud_alloc(struct zbud_pool *pool, size_t size, gfp_t gfp,
-	unsigned long *handle);
-void zbud_free(struct zbud_pool *pool, unsigned long handle);
-int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries);
-void *zbud_map(struct zbud_pool *pool, unsigned long handle);
-void zbud_unmap(struct zbud_pool *pool, unsigned long handle);
-u64 zbud_get_pool_size(struct zbud_pool *pool);
-
-#endif /* _ZBUD_H_ */

mm/Kconfig

@@ -674,6 +674,7 @@ config ZPOOL
 
 config ZBUD
 	tristate "Low (Up to 2x) density storage for compressed pages"
+	depends on ZPOOL
 	help
 	  A special purpose allocator for storing compressed pages.
 	  It is designed to store up to two compressed pages per physical