Don't clear "we have offset" flag even if long is overflow for fseek(),

there is no harm to have it, it will reduce next call efforts.

lib/libc/stdio/fseek.c

@@ -304,7 +304,6 @@ dumb:
 		return (EOF);
 	/* POSIX require long type resulting offset for fseek() */
 	if (ltest && fp->_offset != (long)fp->_offset)  {
-		fp->_flags &= ~__SOFF;
 		errno = EOVERFLOW;
 		return (EOF);
 	}