[Bug 239799] IP_MINTTL broken

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=239799

            Bug ID: 239799
           Summary: IP_MINTTL broken
           Product: Base System
           Version: 12.0-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: vincent at bernat.ch

Hey!

Use of IP_MINTTL to ensure a minimum TTL on input TCP packets is broken. When
this option is enabled for a socket, all incoming packets are dropped. When it
comes to compare the TTL in `tcp_input.c`, the reported TTL is 0:

```c
        if (inp->inp_ip_minttl != 0) {
#ifdef INET6
                if (isipv6) {
                        if (inp->inp_ip_minttl > ip6->ip6_hlim)
                                goto dropunlock;
                } else
#endif
                if (inp->inp_ip_minttl > ip->ip_ttl)
                        goto dropunlock;
        }
```

A few lines earlier, we have:

```c
struct ipovly *ipov = (struct ipovly *)ip;

/*
 * Checksum extended TCP header and data.
 */
len = off0 + tlen;
bzero(ipov->ih_x1, sizeof(ipov->ih_x1));
ipov->ih_len = htons(tlen);
th->th_sum = in_cksum(m, len);
/* Reset length for SDT probes. */
ip->ip_len = htons(len);
/* Reset TOS bits */
ip->ip_tos = iptos;
/* Re-initialization for later version check */
ip->ip_v = IPVERSION;
ip->ip_hl = off0 >> 2;
```

`ip->ip_ttl` also needs to be restored after this code.

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