perf stat: Keep the / modifier separator in fallback

The 'perf stat' fallback for EACCES error sets the exclude_kernel
perf_event_attr and tries perf_event_open() again with it. In addition,
it also changes the name of the event to reflect that change by adding
the 'u' modifier.

But it does not take into account the '/' separator, so the event name
can end up mangled, like: (note the '/:' characters)

  $ perf stat -e cpu/cpu-cycles/ kill
  ...
             386,832      cpu/cpu-cycles/:u

Adding the code to check on the '/' separator and set the following
correct event name:

  $ perf stat -e cpu/cpu-cycles/ kill
  ...
             388,548      cpu/cpu-cycles/u

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180423090823.32309-4-jolsa@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>