[dfs] Change the mode parameter of open to '...'

In the POSIX standard, the prototype of the open function is:
int open(const char *path, int oflag, ... );

components/dfs/include/dfs_posix.h

@@ -119,7 +119,7 @@ int closedir(DIR* d);
 struct stat;
 
 /* file api*/
-int open(const char *file, int flags, int mode);
+int open(const char *file, int flags, ...);
 int close(int d);
 #ifdef RT_USING_NEWLIB
 _READ_WRITE_RETURN_TYPE _EXFUN(read, (int __fd, void *__buf, size_t __nbyte));

components/dfs/src/dfs_posix.c

@@ -41,7 +41,7 @@
  *
  * @return the non-negative integer on successful open, others for failed.
  */
-int open(const char *file, int flags, int mode)
+int open(const char *file, int flags, ...)
 {
     int fd, result;
     struct dfs_fd *d;