mm: showmem: make the notifiers atomic

There are places in kernel like the lowmemorykiller which
invokes show_mem_call_notifiers from an atomic context.
So move from a blocking notifier to atomic. At present
the notifier callbacks does not call sleeping functions,
but it should be made sure, it does not happen in future also.

Change-Id: I9668e67463ab8a6a60be55dbc86b88f45be8b041
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>