Avoid calling CURVNET_[SET|RESTORE] for each packet

The LRO possible test was calling CURVNET_SET once for IPv4 or IPv6 for
each packet in a chain. Only call it once per chain instead.

Submitted by: Matthew Macy <mmacy@mattmacy.io>
Reviewed by: cem, ae
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D13368