perf session: Define 'bytes_transferred' and 'bytes_compressed' metrics

perf session: Define 'bytes_transferred' and 'bytes_compressed' metrics

Define 'bytes_transferred' and 'bytes_compressed' metrics to calculate
ratio in the end of the data collection:

	compression ratio = bytes_transferred / bytes_compressed

The 'bytes_transferred' metric accumulates the amount of bytes that was
extracted from the mmaped kernel buffers for compression, while
'bytes_compressed' accumulates the amount of bytes that was received
after applying compression.

Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1d4bf499-cb03-26dc-6fc6-f14fec7622ce@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>