perf report: Count number of entries separately

perf report: Count number of entries separately

The hists->nr_entries is counted in multiple places so that they can
confuse readers of the code.  This is a preparation of later change
and do not intend any functional difference.

Note that report__collapse_hists() now changed to return nothing since
its return value (nr_samples) is only for checking if there's any data
in the input file and this can be acheived by checking ->nr_entries.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1398327843-31845-2-git-send-email-namhyung@kernel.org
Signed-off-by: Jiri Olsa <jolsa@kernel.org>