perf hists: Rename total_session to total_period

Nowadays we do it per evsel, not per session (that may have multiple
evsels), so rename it to avoid confusion.

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-azsgomr5h4dmaudoogw48w49@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>