fix tun_platform_write return value on FreeBSD: subtract 4-byte AF_INET header from writev result

src/tun_freebsd.c

@@ -356,7 +356,8 @@ ssize_t tun_platform_write(struct tun_if* tun, const uint8_t* buf, size_t len) {
     iv[0].iov_len = sizeof(family);
     iv[1].iov_base = (void*)buf;
     iv[1].iov_len = len;
-    return writev(tun->fd, iv, 2);
+    ssize_t wr = writev(tun->fd, iv, 2);
+    return wr > 0 ? wr - 4 : wr;   // вычесть 4-байтовый AF_INET заголовок
 }
 // Get poll fd for uasync
 int tun_platform_get_poll_fd(struct tun_if* tun) {