f2fs: introduce mode=lfs mount option

This mount option is to enable original log-structured filesystem forcefully.
So, there should be no random writes for main area.

Especially, this supports host-managed SMR device.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/recovery.c

@@ -624,8 +624,12 @@ out:
 	if (err) {
 		bool invalidate = false;
 
-		if (discard_next_dnode(sbi, blkaddr))
+		if (test_opt(sbi, LFS)) {
+			update_meta_page(sbi, NULL, blkaddr);
 			invalidate = true;
+		} else if (discard_next_dnode(sbi, blkaddr)) {
+			invalidate = true;
+		}
 
 		/* Flush all the NAT/SIT pages */
 		while (get_pages(sbi, F2FS_DIRTY_META))