jffs2: typo in comment

It says FB instead of FS (file system).
Signed-off-by: NDan Carpenter <error27@gmail.com>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>

fs/jffs2/jffs2_fs_sb.h

@@ -144,4 +144,4 @@ struct jffs2_sb_info {
 	void *os_priv;
 };
 
-#endif /* _JFFS2_FB_SB */
+#endif /* _JFFS2_FS_SB */