f2fs: merge read IOs at ra_nat_pages()

Change log from v1:
  o add mark_page_accessed() not to reclaim the nat pages.

This patch changes the policy of submitting read bios at ra_nat_pages.

Previously, f2fs submits small read bios with block plugging.
But, with this patch, f2fs itself merges read bios first and then submits a
large bio, which can reduce the bio handling overheads.
Signed-off-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/node.c

@@ -89,13 +89,10 @@ static void ra_nat_pages(struct f2fs_sb_info *sbi, int nid)
 {
 	struct address_space *mapping = sbi->meta_inode->i_mapping;
 	struct f2fs_nm_info *nm_i = NM_I(sbi);
-	struct blk_plug plug;
 	struct page *page;
 	pgoff_t index;
 	int i;
 
-	blk_start_plug(&plug);
-
 	for (i = 0; i < FREE_NID_PAGES; i++, nid += NAT_ENTRY_PER_BLOCK) {
 		if (nid >= nm_i->max_nid)
 			nid = 0;
@@ -105,15 +102,15 @@ static void ra_nat_pages(struct f2fs_sb_info *sbi, int nid)
 		if (!page)
 			continue;
 		if (PageUptodate(page)) {
+			mark_page_accessed(page);
 			f2fs_put_page(page, 1);
 			continue;
 		}
-		if (f2fs_readpage(sbi, page, index, READ))
-			continue;
-
+		submit_read_page(sbi, page, index, READ_SYNC);
+		mark_page_accessed(page);
 		f2fs_put_page(page, 0);
 	}
-	blk_finish_plug(&plug);
+	f2fs_submit_read_bio(sbi, READ_SYNC);
 }
 
 static struct nat_entry *__lookup_nat_cache(struct f2fs_nm_info *nm_i, nid_t n)