perf test: Fix dso cache testcase

perf test: Fix dso cache testcase

The current dso cache permits to keep dso->data.fd is open under a half
of open file limit.  But test__dso_data_cache() sets dso_cnt to limit /
2 + 1 so it'll reach the limit in the loop even though the loop count is
one less than the dso_cnt and it makes the final dso__data_fd() after
the loop meaningless.

I guess the intention was dsos[0]->data.fd is open before the last open
and gets closed after it.  So add an assert before the last open.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1422585209-32742-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>