f2fs: read node blocks ahead when truncating blocks

This patch enables reading node blocks in advance when truncating large
data blocks.

 > time rm $MNT/testfile (500GB) after drop_cachees
Before : 9.422 s
After  : 4.821 s

Reported-by: Stephen Bates <stephen.bates@microsemi.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index fb8cc27..3146129 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -584,7 +584,7 @@
 	}
 
 	set_new_dnode(&dn, inode, ipage, NULL, 0);
-	err = get_dnode_of_data(&dn, free_from, LOOKUP_NODE);
+	err = get_dnode_of_data(&dn, free_from, LOOKUP_NODE_RA);
 	if (err) {
 		if (err == -ENOENT)
 			goto free_next;