syscall: fix hang when using Unshareflags: CLONE_NEWUSER with uid/gid mapping (linux)

Note that this particular combination of properties still fails (EPERM), but it
no longer hangs.

Updates #29789

Change-Id: I29b15b85a25a7acd7ae89ffc5fed074bcdfe0a12
Reviewed-on: https://go-review.googlesource.com/c/go/+/158297
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/exec_linux.go

@@ -154,7 +154,7 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 	runtime_BeforeFork()
 	locked = true
 	switch {
-	case runtime.GOARCH == "amd64" && sys.Cloneflags&CLONE_NEWUSER == 0:
+	case runtime.GOARCH == "amd64" && (sys.Cloneflags&CLONE_NEWUSER == 0 && sys.Unshareflags&CLONE_NEWUSER == 0):
 		r1, err1 = rawVforkSyscall(SYS_CLONE, uintptr(SIGCHLD|CLONE_VFORK|CLONE_VM)|sys.Cloneflags)
 	case runtime.GOARCH == "s390x":
 		r1, _, err1 = RawSyscall6(SYS_CLONE, 0, uintptr(SIGCHLD)|sys.Cloneflags, 0, 0, 0, 0)