lowmemorykiller: ignore the tasks that has null mm for reaping

The setting of task_struct->mm to NULL might race when the same process
gets selected in the lowmemorykiller path for reaping of memory.So,
ignore the process for reaping when its mm NULL.

Change-Id: I40a50cc2f6dea892ed9b31df7aad59a349ff8c9c
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>