perf c2c: Fix report with offline cpus

If c2c is recorded on a machine where any cpus are offline, 'perf c2c
report' throws an error "node/cpu topology bugFailed setup nodes".

It fails because while preparing node-cpu mapping we don't consider
offline cpus.

Reported-by: Nageswara R Sastry <nasastry@in.ibm.com>
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Fixes: 1e181b92

 ("perf c2c report: Add 'node' sort key")
Link: http://lkml.kernel.org/r/20190822085045.25108-1-ravi.bangoria@linux.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>