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/f2fs.h b/fs/f2fs/f2fs.h
index aad4720..f05ae22 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -250,6 +250,7 @@
 #define F2FS_GOING_DOWN_FULLSYNC	0x0	/* going down with full sync */
 #define F2FS_GOING_DOWN_METASYNC	0x1	/* going down with metadata */
 #define F2FS_GOING_DOWN_NOSYNC		0x2	/* going down */
+#define F2FS_GOING_DOWN_METAFLUSH	0x3	/* going down with meta flush */
 
 #if defined(__KERNEL__) && defined(CONFIG_COMPAT)
 /*