udp_input: remove a BSD stack relict

[glebius]

Summary

I should had removed it 9 years ago in 8ad458a471ca. That commit
left save_ip as a write-only variable.

[ae]

It isn't write only variable. At line 501 original IP header can be changed, this variable keeps its copy.

[glebius]

Thanks Andrew for noticing that checksum calculation actually
alters IP header, despite the char b[9] saver.

[ae]

sys/netinet/udp_usrreq.c

492

What you think, if do it like this:

char b[offsetof(struct ipovly, ih_src)];
struct ipovly *p = (stuct ipovly *)ip;

bcopy(p, b, sizeof(b));
bzero(p, sizeof(p->ih_x1));
p->ih_len = ...
...
bcopy(b, p, sizeof(b));

[glebius]

In D32719#738820, @ae wrote:

It isn't write only variable. At line 501 original IP header can be changed, this variable keeps its copy.

There is another saver there - the char b[9]. It saves everything and restores back. The only value thrashed

sys/netinet/udp_usrreq.c

492

Great idea, thanks!

[glebius]

Implement as Andrey suggests.

[zlei]

sys/netinet/udp_usrreq.c

492

offsetof(struct ipovly, ih_src) is actually 12, not the same with the original hardcode 9. Is this intentional ?