Avoid calling CURVNET_[SET|RESTORE] up to twice per packet

[shurd]

Summary

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

Test Plan

Check for improved performance

[sbruno]

Spicy

[cem]

sys/net/iflib.c

2471

I would call this kind of operation "get" rather than "set"

2475

Also spelled *v6 = (VNET(ip6_forwarding) == 0);

[ae]

sys/net/iflib.c

2474

Both conditions should be guarded with #ifdef INET and #ifdef INET6.

[shurd]

Integrate feedback from ae and cem

Rename functions and variables to better indicate what the represent
rather than what they're used for. Simplify logic, check INET and INET6
definitions.

[shurd]

Updated, thanks.

[cem]

A static analyzer may reject passing uninitialized values to iflib_check_lro_possible in the case that one of INET or INET6 is disabled. But I don't think there is any real problem as the uninitialized value is unused.