netinet: re-read IP length after PFIL hook

[kp]

Summary

The pfil hook may modify the packet, so before we check its length (to
decide if it needs to be fragmented or not) we should re-read that
length.

This is most likely to happen when pf is reassembling packets. In that
scenario we'd receive the last fragment, which is likely to be a short
packet, pf would reassemble it (likely exceeding the interface MTU) and
then we'd transmit it without fragmenting, because we're comparing the
MTU to the length of the last fragment, not the fully reassembled
packet.

See also: https://redmine.pfsense.org/issues/14396
MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")

[mjg]

so the reload is only needed if hooks are present to begin with? as in it should get hoisted up

what about other fields:

ip = mtod(m, struct ip *);
ip_len = ntohs(ip->ip_len);
ip_off = ntohs(ip->ip_off);

ip is is already getting reloaded

[kp]

• move re-read of ip_len to only be done when we run pfil hooks.

[cy]

LGTM