magicsock data race

[danderson]

Found by CI. Triggers infrequently.

2020-03-25T17:45:24.3688273Z redo    oss/wgengine/magicsock/testrace
2020-03-25T17:45:24.3688618Z ==================
2020-03-25T17:45:24.3688963Z WARNING: DATA RACE
2020-03-25T17:45:24.3689322Z Read at 0x00c00007cc80 by goroutine 75:
2020-03-25T17:45:24.3690642Z   github.com/tailscale/wireguard-go/device.deviceUpdateState()
2020-03-25T17:45:24.3691447Z       /home/runner/work/corp/corp/wireguard-go/device/device.go:163 +0x38e
2020-03-25T17:45:24.3692102Z   github.com/tailscale/wireguard-go/device.(*Device).Up()
2020-03-25T17:45:24.3692942Z       /home/runner/work/corp/corp/wireguard-go/device/device.go:204 +0x79
2020-03-25T17:45:24.3693645Z   github.com/tailscale/wireguard-go/device.(*Device).RoutineTUNEventReader()
2020-03-25T17:45:24.3694303Z       /home/runner/work/corp/corp/wireguard-go/device/tun.go:44 +0x30c
2020-03-25T17:45:24.3694617Z 
2020-03-25T17:45:24.3695003Z Previous write at 0x00c00007cc80 by goroutine 57:
2020-03-25T17:45:24.3695619Z   github.com/tailscale/wireguard-go/device.(*Device).Reconfig()
2020-03-25T17:45:24.3696508Z       /home/runner/work/corp/corp/wireguard-go/device/config.go:124 +0xc61
2020-03-25T17:45:24.3697362Z   tailscale.com/wgengine/magicsock.TestTwoDevicePing()
2020-03-25T17:45:24.3697882Z       /home/runner/work/corp/corp/oss/wgengine/magicsock/magicsock_test.go:327 +0xf8e
2020-03-25T17:45:24.3698298Z   testing.tRunner()
2020-03-25T17:45:24.3698779Z       /opt/hostedtoolcache/go/1.13.7/x64/src/testing/testing.go:909 +0x199
2020-03-25T17:45:24.3699091Z 
2020-03-25T17:45:24.3699497Z Goroutine 75 (running) created at:
2020-03-25T17:45:24.3700173Z   github.com/tailscale/wireguard-go/device.NewDevice()
2020-03-25T17:45:24.3700939Z       /home/runner/work/corp/corp/wireguard-go/device/device.go:405 +0x61d
2020-03-25T17:45:24.3701456Z   tailscale.com/wgengine/magicsock.TestTwoDevicePing()
2020-03-25T17:45:24.3701967Z       /home/runner/work/corp/corp/oss/wgengine/magicsock/magicsock_test.go:320 +0xf5a
2020-03-25T17:45:24.3702398Z   testing.tRunner()
2020-03-25T17:45:24.3702865Z       /opt/hostedtoolcache/go/1.13.7/x64/src/testing/testing.go:909 +0x199
2020-03-25T17:45:24.3703188Z 
2020-03-25T17:45:24.3703599Z Goroutine 57 (running) created at:
2020-03-25T17:45:24.3703997Z   testing.(*T).Run()
2020-03-25T17:45:24.3704827Z       /opt/hostedtoolcache/go/1.13.7/x64/src/testing/testing.go:960 +0x651
2020-03-25T17:45:24.3705326Z   testing.runTests.func1()
2020-03-25T17:45:24.3706515Z       /opt/hostedtoolcache/go/1.13.7/x64/src/testing/testing.go:1202 +0xa6
2020-03-25T17:45:24.3707062Z   testing.tRunner()
2020-03-25T17:45:24.3707618Z       /opt/hostedtoolcache/go/1.13.7/x64/src/testing/testing.go:909 +0x199
2020-03-25T17:45:24.3708029Z   testing.runTests()
2020-03-25T17:45:24.3708581Z       /opt/hostedtoolcache/go/1.13.7/x64/src/testing/testing.go:1200 +0x521
2020-03-25T17:45:24.3709065Z   testing.(*M).Run()
2020-03-25T17:45:24.3710332Z       /opt/hostedtoolcache/go/1.13.7/x64/src/testing/testing.go:1117 +0x2ff
2020-03-25T17:45:24.3710773Z   main.main()
2020-03-25T17:45:24.3711174Z       _testmain.go:54 +0x223
2020-03-25T17:45:24.3711593Z ==================