perf bench numa: Fixup discontiguous/sparse numa nodes

Certain systems are designed to have sparse/discontiguous nodes.  On
such systems, 'perf bench numa' hangs, shows wrong number of nodes and
shows values for non-existent nodes. Handle this by only taking nodes
that are exposed by kernel to userspace.

Signed-off-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1edbcd353c009e109e93d78f2f46381930c340fe.1511368645.git.sathnaga@linux.vnet.ibm.com


Signed-off-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>