lowmemorykiller: Dump out slab state information

Leaks from the slab allocator are a very common cause of
out of memory issues and excessive lowmemorykiller killing.
Dump out the slab statistics as well to help with debugging.

Change-Id: Ia5e4eeb66092c76f2b761321c88fa75c4138927c
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>