perf report: Add option to show total period

Just like --show-nr-samples, to help in diagnosing problems in the
tools.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-1lr7ejdjfvy2uwy2wkmatcpq@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>