f2fs: add F2FS_GOING_DOWN_METAFLUSH to test power-failure

This patch introduces F2FS_GOING_DOWN_METAFLUSH which flushes meta pages like
SSA blocks and then blocks all the writes.
This can be used by power-failure tests.

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index b3985a6..6d3cfd5 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -1498,6 +1498,10 @@
 	case F2FS_GOING_DOWN_NOSYNC:
 		f2fs_stop_checkpoint(sbi);
 		break;
+	case F2FS_GOING_DOWN_METAFLUSH:
+		sync_meta_pages(sbi, META, LONG_MAX);
+		f2fs_stop_checkpoint(sbi);
+		break;
 	default:
 		return -EINVAL;
 	}