mm: declare some external symbols

Declare 'bdi_pending_list' and 'tag_pages_for_writeback()' to remove
following sparse warnings:

 mm/backing-dev.c:46:1: warning: symbol 'bdi_pending_list' was not declared. Should it be static?
 mm/page-writeback.c:825:6: warning: symbol 'tag_pages_for_writeback' was not declared. Should it be static?
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/backing-dev.h

@@ -111,6 +111,7 @@ void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi);
 
 extern spinlock_t bdi_lock;
 extern struct list_head bdi_list;
+extern struct list_head bdi_pending_list;
 
 static inline int wb_has_dirty_io(struct bdi_writeback *wb)
 {

include/linux/writeback.h

@@ -149,6 +149,8 @@ int write_cache_pages(struct address_space *mapping,
 int do_writepages(struct address_space *mapping, struct writeback_control *wbc);
 void set_page_dirty_balance(struct page *page, int page_mkwrite);
 void writeback_set_ratelimit(void);
+void tag_pages_for_writeback(struct address_space *mapping,
+			     pgoff_t start, pgoff_t end);
 
 /* pdflush.c */
 extern int nr_pdflush_threads;	/* Global so it can be exported to sysctl