libct/config: fix a data race

As reported by go test -race ./libcontainer/configs:

=== RUN   TestCommandHookRunTimeout
==================
WARNING: DATA RACE
Read at 0x00c000202230 by goroutine 23:
  os/exec.(*Cmd).Wait()
      /usr/lib/golang/src/os/exec/exec.go:502 +0x91
  github.com/opencontainers/runc/libcontainer/configs.Command.Run()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/configs/config.go:390 +0x58c
  github.com/opencontainers/runc/libcontainer/configs_test.TestCommandHookRunTimeout()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/configs/config_test.go:223 +0x3ed
  testing.tRunner()
      /usr/lib/golang/src/testing/testing.go:1123 +0x202

Previous write at 0x00c000202230 by goroutine 27:
  os/exec.(*Cmd).Wait()
      /usr/lib/golang/src/os/exec/exec.go:505 +0xb4
  github.com/opencontainers/runc/libcontainer/configs.Command.Run.func1()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/configs/config.go:373 +0x55

Goroutine 23 (running) created at:
  testing.(*T).Run()
      /usr/lib/golang/src/testing/testing.go:1168 +0x5bb
  testing.runTests.func1()
      /usr/lib/golang/src/testing/testing.go:1439 +0xa6
  testing.tRunner()
      /usr/lib/golang/src/testing/testing.go:1123 +0x202
  testing.runTests()
      /usr/lib/golang/src/testing/testing.go:1437 +0x612
  testing.(*M).Run()
      /usr/lib/golang/src/testing/testing.go:1345 +0x3b3
  main.main()
      _testmain.go:69 +0x236

Goroutine 27 (running) created at:
  github.com/opencontainers/runc/libcontainer/configs.Command.Run()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/configs/config.go:372 +0x415
  github.com/opencontainers/runc/libcontainer/configs_test.TestCommandHookRunTimeout()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/configs/config_test.go:223 +0x3ed
  testing.tRunner()
      /usr/lib/golang/src/testing/testing.go:1123 +0x202
==================
    testing.go:1038: race detected during execution of test
--- FAIL: TestCommandHookRunTimeout (0.10s)

Apparently, the issue is we call two Wait()s for the same command
which can race internally.

Fix is easy -- since we already have a waiting goroutine,
wait for it to return instead of calling a second Wait().

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/configs/config.go

@@ -387,7 +387,7 @@ func (c Command) Run(s *specs.State) error {
 		return err
 	case <-timerCh:
 		cmd.Process.Kill()
-		cmd.Wait()
+		<-errC
 		return fmt.Errorf("hook ran past specified timeout of %.1fs", c.Timeout.Seconds())
 	}
 }