perf bench mem: Fix 'length' vs. 'size' naming confusion

So 'perf bench mem memcpy/memset' consistently uses 'len' and 'length'
for buffer sizes - while it's really a memory buffer size. (strings have
length.)

Rename all affected variables.

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/1445241870-24854-10-git-send-email-mingo@kernel.org


[ Update perf-bench man page ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>