mm, oom: Try to online memory block before killing

Before killing a process first try to see if there are any offlined
memory blocks, if there are try to online one of them.

Change-Id: Ie97c8b69f0ba173c202a891a38a5c914869ddaae
Signed-off-by: Liam Mark <lmark@codeaurora.org>
[isaacm@codeaurora.org: Update walk_memory_range to walk_memory_blocks]
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>