perf hists: Print number of samples, not the period sum

So that we match the header where we state the number of events with the
"Samples" column when using 'perf report -n/--show-nr-samples':

 [root@emilia ~]# perf record -a sleep 1
 [ perf record: Woken up 1 times to write data ]
 [ perf record: Captured and wrote 0.111 MB perf.data (~4860 samples) ]
 [root@emilia ~]# perf report --stdio --show-nr-samples
 # Events: 11  cycles
 #
 # Overhead  Samples        Command       Shared Object                        Symbol
 # ........ ..........  ...........  ..................  ............................
 #
     16.65%          1        sleep  [kernel.kallsyms]   [k] unmap_vmas
     16.10%          1         perf  libpthread-2.12.so  [.] __pthread_cleanup_push_defer
     15.79%          2         perf  [kernel.kallsyms]   [k] format_decode
     12.88%          1  kworker/1:2  [kernel.kallsyms]   [k] cache_reap
     10.69%          1      swapper  [kernel.kallsyms]   [k] _raw_spin_lock
      7.55%          1        sleep  [kernel.kallsyms]   [k] prepare_exec_creds
      6.00%          1         perf  [jbd2]              [k] start_this_handle
      5.29%          1         perf  [kernel.kallsyms]   [k] seq_read
      4.75%          1         perf  [kernel.kallsyms]   [k] get_pid_task
      4.30%          1         perf  [kernel.kallsyms]   [k] _raw_spin_unlock_irqrestore

 #
 # (For a higher level overview, try: perf report --sort comm,dso)
 #
 [root@emilia ~]#

Reported-by: Stephane Eranian <eranian@google.com>
Reported-by: Cliff Wickman <cpw@sgi.com>
Acked-by: Stephane Eranian <eranian@google.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: <stable@kernel.org>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
[ cherry-picked it from perf/core, as it has been reported by others as well. ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>