staging: android/lowmemorykiller: Don't unregister notifier from atomic context

The lowmemorykiller registers an atomic notifier for notfication of when
the task is freed.  From this atomic notifier callback, it removes the
atomic notifier via task_free_unregister().  This is incorrect because
atomic_notifier_chain_unregister() calls syncronize_rcu(), which can
sleep, which shouldn't be done from an atomic notifier.

Fix this by registering the notifier during init, and only unregister it
if the lowmemorykiller is unloaded.

Rebased to -next by Paul E. McKenney.
Rebased to -next again by Anton Vorontsov.

Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Christian Bejram <christian.bejram@stericsson.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reported-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Change-Id: I649bcd173d3a9e13a600b295cc1e5c8402f06083