MFC 290028:
Turning on IPSEC used to introduce a slight amount of performance
degradation (7%) for host host TCP connections over 10Gbps links,
even when there were no secuirty policies in place. There is no
change in performance on 1Gbps network links. Testing GENERIC vs.
GENERIC-NOIPSEC vs. GENERIC with this change shows that the new
code removes any overhead introduced by having IPSEC always in the
kernel.

Differential Revision: D3993
Sponsored by: Rubicon Communications (Netgate)

MFC r275392:
Remove route chaching support from ipsec code. It isn't used for some time.
* remove sa_route_union declaration and route_cache member from struct secashead;
* remove key_sa_routechange() call from ICMP and ICMPv6 code;
* simplify ip_ipsec_mtu();
* remove #include <net/route.h>;

Sponsored by: Yandex LLC

Merge r262763, r262767, r262771, r262806 from head:
- Remove rt_metrics_lite and simply put its members into rtentry.
- Use counter(9) for rt_pksent (former rt_rmx.rmx_pksent). This
removes another cache trashing ++ from packet forwarding path.
- Create zini/fini methods for the rtentry UMA zone. Via initialize
mutex and counter in them.
- Fix reporting of rmx_pksent to routing socket.
- Fix netstat(1) to report "Use" both in kvm(3) and sysctl(3) mode.

MFC r275392:
Remove route chaching support from ipsec code. It isn't used for some time.
* remove sa_route_union declaration and route_cache member from struct secashead;
* remove key_sa_routechange() call from ICMP and ICMPv6 code;
* simplify ip_ipsec_mtu();
* remove #include <net/route.h>;

Sponsored by: Yandex LLC

Merge r262763, r262767, r262771, r262806 from head:
- Remove rt_metrics_lite and simply put its members into rtentry.
- Use counter(9) for rt_pksent (former rt_rmx.rmx_pksent). This
removes another cache trashing ++ from packet forwarding path.
- Create zini/fini methods for the rtentry UMA zone. Via initialize
mutex and counter in them.
- Fix reporting of rmx_pksent to routing socket.
- Fix netstat(1) to report "Use" both in kvm(3) and sysctl(3) mode.