Initialize _offset to 0 in fopen(), it helps to optimize fseek/ftell

lib/libc/stdio/fopen.c

@@ -69,6 +69,8 @@ fopen(file, mode)
 	}
 	fp->_file = f;
 	fp->_flags = flags;
+	fp->_flags |= __SOFF;
+	fp->_offset = 0;
 	fp->_cookie = fp;
 	fp->_read = __sread;
 	fp->_write = __swrite;