ptrace: Kill task with -9 explicitly when detaching

Strange fact, but a task seems to live for some time after PTRACE_KILL :(
Don't know yet why this happens, I'm poking Oleg on that, but this one makes
it die right at once.

The streaming file read test passes after this.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>