perf bench: Remove the prefaulting complication from 'perf bench mem mem*'

So 'perf bench mem memcpy/memset' has elaborate code to measure
memcpy()/memset() performance both with freshly allocated buffers (which
includes initial page fault overhead) and with preallocated buffers.

But the thing is, the resulting bandwidth results are mostly
meaningless, because page faults dominate so much of the cost.

It might make sense to measure cache cold vs. cache hot performance, but
the code does not do this.

So remove this complication, and always prefault the ranges before using
them.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1445241870-24854-6-git-send-email-mingo@kernel.org
[ Remove --no-prefault, --only-prefault from docs, noticed by David Ahern ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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