Add necessary flag to mark file as being opened.

lfs.h

@@ -136,6 +136,7 @@ enum lfs_open_flags {
     LFS_F_READING = 0x040000, // File has been read since last flush
     LFS_F_ERRED   = 0x080000, // An error occured during write
     LFS_F_INLINE  = 0x100000, // Currently inlined in directory entry
+    LFS_F_OPENED  = 0x200000, // File has been opened
 };
 
 // File seek flags