perf bench: Add -t/--threads option to perf bench mem mmap

perf bench: Add -t/--threads option to perf bench mem mmap

So that it can measure overhead of mmap_lock and/or per-VMA lock
contention.

  $ perf bench mem mmap -f demand -l 1000 -t 1
  # Running 'mem/mmap' benchmark:
  # function 'demand' (Demand loaded mmap())
  # Copying 1MB bytes ...

         2.786858 GB/sec

  $ perf bench mem mmap -f demand -l 1000 -t 2
  # Running 'mem/mmap' benchmark:
  # function 'demand' (Demand loaded mmap())
  # Copying 1MB bytes ...

         1.624468 GB/sec/thread   ( +-   0.30% )

  $ perf bench mem mmap -f demand -l 1000 -t 3
  # Running 'mem/mmap' benchmark:
  # function 'demand' (Demand loaded mmap())
  # Copying 1MB bytes ...

         1.493068 GB/sec/thread   ( +-   0.15% )

  $ perf bench mem mmap -f demand -l 1000 -t 4
  # Running 'mem/mmap' benchmark:
  # function 'demand' (Demand loaded mmap())
  # Copying 1MB bytes ...

         1.006087 GB/sec/thread   ( +-   0.41% )

Reviewed-by: Ankur Arora <ankur.a.arora@oracle.com>
Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>