perf tests: Avoid possible truncation with dirent->d_name + snprintf

perf tests: Avoid possible truncation with dirent->d_name + snprintf

Addressing a few cases spotted by a new warning in gcc 7:

  tests/parse-events.c: In function 'test_pmu_events':
  tests/parse-events.c:1790:39: error: '%s' directive output may be truncated writing up to 255 bytes into a region of size 90 [-Werror=format-truncation=]
     snprintf(name, MAX_NAME, "cpu/event=%s/u", ent->d_name);
                                       ^~
  In file included from /usr/include/stdio.h:939:0,
                   from /git/linux/tools/perf/util/map.h:9,
                   from /git/linux/tools/perf/util/symbol.h:7,
                   from /git/linux/tools/perf/util/evsel.h:10,
                   from tests/parse-events.c:3:
  /usr/include/bits/stdio2.h:64:10: note: '__builtin___snprintf_chk' output between 13 and 268 bytes into a destination of size 100
     return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          __bos (__s), __fmt, __va_arg_pack ());
          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  tests/parse-events.c:1798:29: error: '%s' directive output may be truncated writing up to 255 bytes into a region of size 100 [-Werror=format-truncation=]
     snprintf(name, MAX_NAME, "%s:u,cpu/event=%s/u", ent->d_name, ent->d_name);

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 945aea220bb8 ("perf tests: Move test objects into 'tests' directory")
Link: http://lkml.kernel.org/n/tip-ty4q2p8zp1dp3mskvubxskm5@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>