perf bench mem: Add mmap() workloads

Add two mmap() workloads: one that eagerly populates a region and
another that demand faults it in.

The intent is to probe the memory subsytem performance incurred
by mmap().

  $ perf bench mem mmap -s 4gb -p 4kb -l 10 -f populate
  # Running 'mem/mmap' benchmark:
  # function 'populate' (Eagerly populated map())
  # Copying 4gb bytes ...

       1.811691 GB/sec

  $ perf bench mem mmap -s 4gb -p 2mb -l 10 -f populate
  # Running 'mem/mmap' benchmark:
  # function 'populate' (Eagerly populated mmap())
  # Copying 4gb bytes ...

      12.272017 GB/sec

  $ perf bench mem mmap -s 4gb -p 1gb -l 10 -f populate
  # Running 'mem/mmap' benchmark:
  # function 'populate' (Eagerly populated mmap())
  # Copying 4gb bytes ...

      17.085927 GB/sec

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: Namhyung Kim <namhyung@kernel.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>