perf report: Report number of events, not samples

Number of samples is meaningless after we switched to auto-freq, so
report the number of events, i.e. not the sum of the different periods,
but the number PERF_RECORD_SAMPLE emitted by the kernel.

While doing this I noticed that naming "count" to the sum of all the
event periods can be confusing, so rename it to .period, just like in
struct sample.data, so that we become more consistent.

This helps with the next step, that was to record in struct hist_entry
the number of sample events for each instance, we need that because we
use it to generate the number of events when applying filters to the
tree of hist entries like it is being done in the TUI report browser.

Suggested-by: Ingo Molnar <mingo@elte.hu>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>