This avoids a non-obvious data race, where the JSON decoder ends
up creating do-nothing writes into global variables.
==================
WARNING: DATA RACE
Write at 0x0000011e1860 by goroutine 201:
tailscale.com/wgengine/packet.(*IP).UnmarshalJSON()
/home/crawshaw/repo/corp/oss/wgengine/packet/packet.go:83 +0x2d9
encoding/json.(*decodeState).literalStore()
/home/crawshaw/go/go/src/encoding/json/decode.go:877 +0x445e
...
encoding/json.Unmarshal()
/home/crawshaw/go/go/src/encoding/json/decode.go:107 +0x1de
tailscale.com/control/controlclient.(*Direct).decodeMsg()
/home/crawshaw/repo/corp/oss/control/controlclient/direct.go:615 +0x1ab
tailscale.com/control/controlclient.(*Direct).PollNetMap()
/home/crawshaw/repo/corp/oss/control/controlclient/direct.go:525 +0x1053
tailscale.com/control/controlclient.(*Client).mapRoutine()
/home/crawshaw/repo/corp/oss/control/controlclient/auto.go:428 +0x3a6
Previous read at 0x0000011e1860 by goroutine 86:
tailscale.com/wgengine/filter.matchIPWithoutPorts()
/home/crawshaw/repo/corp/oss/wgengine/filter/match.go:108 +0x91
tailscale.com/wgengine/filter.(*Filter).runIn()
/home/crawshaw/repo/corp/oss/wgengine/filter/filter.go:147 +0x3c6
tailscale.com/wgengine/filter.(*Filter).RunIn()
/home/crawshaw/repo/corp/oss/wgengine/filter/filter.go:127 +0xb0
tailscale.com/wgengine.(*userspaceEngine).SetFilter.func1()
/home/crawshaw/repo/corp/oss/wgengine/userspace.go:390 +0xfc
github.com/tailscale/wireguard-go/device.(*Device).RoutineDecryption()
/home/crawshaw/repo/corp/wireguard-go/device/receive.go:295 +0xa1f
For #112
Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
