staging: android: lowmemorykiller: set TIF_MEMDIE before send kill sig

Set TIF_MEMDIE tsk_thread flag before send kill signal to the
selected thread. This is to fit a usual code sequence and avoid
potential race issue.

Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>