slub: Add method to verify memory is not freed
This is for tracking down suspect memory usage.
slub: reduce overhead of slub_debug
slub checks for poison one byte by one, which is highly inefficient and shows up frequently as a highest cpu-eater in perf top. Joining reads gives nice speedup: (Compiling some project with different options) make -j12 make clean slub_debug disabled: 1m 27s 1.2 s slub_debug enabled: 1m 46s 7.6 s slub_debug enabled + this patch: 1m 33s 3.2 s check_bytes still shows up high, but not always at the top.