f2fs: introduce ra_meta_pages to readahead CP/NAT/SIT pages

This patch help us to cleanup the readahead code by merging ra_{sit,nat}_pages
function into ra_meta_pages.
Additionally the new function is used to readahead cp block in
recover_orphan_inodes.

Change log from v1:
 o fix a deadloop bug pointed by Jaegeuk Kim.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
index 82f4753..7689f91 100644
--- a/fs/f2fs/node.c
+++ b/fs/f2fs/node.c
@@ -82,42 +82,6 @@
 	return dst_page;
 }
 
-/*
- * Readahead NAT pages
- */
-static void ra_nat_pages(struct f2fs_sb_info *sbi, int nid)
-{
-	struct address_space *mapping = META_MAPPING(sbi);
-	struct f2fs_nm_info *nm_i = NM_I(sbi);
-	struct page *page;
-	pgoff_t index;
-	int i;
-	struct f2fs_io_info fio = {
-		.type = META,
-		.rw = READ_SYNC | REQ_META | REQ_PRIO
-	};
-
-
-	for (i = 0; i < FREE_NID_PAGES; i++, nid += NAT_ENTRY_PER_BLOCK) {
-		if (unlikely(nid >= nm_i->max_nid))
-			nid = 0;
-		index = current_nat_addr(sbi, nid);
-
-		page = grab_cache_page(mapping, index);
-		if (!page)
-			continue;
-		if (PageUptodate(page)) {
-			mark_page_accessed(page);
-			f2fs_put_page(page, 1);
-			continue;
-		}
-		f2fs_submit_page_mbio(sbi, page, index, &fio);
-		mark_page_accessed(page);
-		f2fs_put_page(page, 0);
-	}
-	f2fs_submit_merged_bio(sbi, META, READ);
-}
-
 static struct nat_entry *__lookup_nat_cache(struct f2fs_nm_info *nm_i, nid_t n)
 {
 	return radix_tree_lookup(&nm_i->nat_root, n);
@@ -1413,7 +1377,7 @@
 		return;
 
 	/* readahead nat pages to be scanned */
-	ra_nat_pages(sbi, nid);
+	ra_meta_pages(sbi, NAT_BLOCK_OFFSET(nid), FREE_NID_PAGES, META_NAT);
 
 	while (1) {
 		struct page *page = get_current_nat_page(sbi, nid);