fs/vfs/fs_open.c: Fix error in nx_vopen() function. 'va_start' is used in function with fixed args. 'va_start'/'va_end' should not be used in nx_vopen function, because it has fixed arguments.

fs/vfs/fs_open.c

@@ -120,9 +120,7 @@ int nx_vopen(FAR const char *path, int oflags, va_list ap)
 
   if ((oflags & (O_WRONLY | O_CREAT)) != 0)
     {
-      va_start(ap, oflags);
       mode = va_arg(ap, mode_t);
-      va_end(ap);
     }
 #endif