Staging: android: lowmemorykiller: fix possible android low memory killer NULL pointer

get_mm_rss() atomically dereferences the actual without checking for a
NULL pointer, which is possible since task_lock() is not held.

Cc: San Mehat <san@android.com>
Cc: Arve Hjønnevåg <arve@android.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>