Export __set_page_dirty

XFS currently contains a copy-and-paste of __set_page_dirty().  Export
it from buffer.c instead.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_aops.c

@@ -1472,19 +1472,8 @@ xfs_vm_set_page_dirty(
 	newly_dirty = !TestSetPageDirty(page);
 	spin_unlock(&mapping->private_lock);
 
-	if (newly_dirty) {
-		/* sigh - __set_page_dirty() is static, so copy it here, too */
-		unsigned long flags;
-
-		spin_lock_irqsave(&mapping->tree_lock, flags);
-		if (page->mapping) {	/* Race with truncate? */
-			WARN_ON_ONCE(!PageUptodate(page));
-			account_page_dirtied(page, mapping);
-			radix_tree_tag_set(&mapping->page_tree,
-					page_index(page), PAGECACHE_TAG_DIRTY);
-		}
-		spin_unlock_irqrestore(&mapping->tree_lock, flags);
-	}
+	if (newly_dirty)
+		__set_page_dirty(page, mapping, 1);
 	unlock_page_memcg(page);
 	if (newly_dirty)
 		__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);