f2fs: add a tracepoint for f2fs_read_data_pages

This patch adds a tracepoint for f2fs_read_data_pages to trace when pages
are readahead by VFS.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -1032,6 +1032,9 @@ static int f2fs_read_data_pages(struct file *file,
 			struct list_head *pages, unsigned nr_pages)
 {
 	struct inode *inode = file->f_mapping->host;
+	struct page *page = list_entry(pages->prev, struct page, lru);
+
+	trace_f2fs_readpages(inode, page, nr_pages);
 
 	/* If the file has inline data, skip readpages */
 	if (f2fs_has_inline_data(inode))