f2fs: add feature facility in superblock

This patch introduces a feature in superblock, which will indicate any new
features for f2fs.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/include/linux/f2fs_fs.h b/include/linux/f2fs_fs.h
index 8d345c2..d44e97f 100644
--- a/include/linux/f2fs_fs.h
+++ b/include/linux/f2fs_fs.h
@@ -90,7 +90,8 @@
 	__le32 cp_payload;
 	__u8 version[VERSION_LEN];	/* the kernel version */
 	__u8 init_version[VERSION_LEN];	/* the initial kernel version */
-	__u8 reserved[892];		/* valid reserved region */
+	__le32 feature;			/* defined features */
+	__u8 reserved[888];		/* valid reserved region */
 } __packed;
 
 /*