perf bench mem: Allow chunking on a memory region

There can be a significant gap in memset/memcpy performance depending
on the size of the region being operated on.

With chunk-size=4kb:

  $ echo madvise > /sys/kernel/mm/transparent_hugepage/enabled

  $ perf bench mem memset -p 4kb -k 4kb -s 4gb -l 10 -f x86-64-stosq
  # Running 'mem/memset' benchmark:
  # function 'x86-64-stosq' (movsq-based memset() in arch/x86/lib/memset_64.S)
  # Copying 4gb bytes ...

      13.011655 GB/sec

With chunk-size=1gb:

  $ echo madvise > /sys/kernel/mm/transparent_hugepage/enabled

  $ perf bench mem memset -p 4kb -k 1gb -s 4gb -l 10 -f x86-64-stosq
  # Running 'mem/memset' benchmark:
  # function 'x86-64-stosq' (movsq-based memset() in arch/x86/lib/memset_64.S)
  # Copying 4gb bytes ...

      21.936355 GB/sec

So, allow the user to specify the chunk-size.

The default value is identical to the total size of the region, which
preserves current behaviour.

Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Raghavendra K T <raghavendra.kt@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>