perf ftrace: Fix failure to set cpumask when only one cpu is present

perf ftrace: Fix failure to set cpumask when only one cpu is present

[ Upstream commit cf30ae726c011e0372fd4c2d588466c8b50a8907 ]

The buffer containing the string used to set cpumask is overwritten at
the end of the string later in cpu_map__snprint_mask due to not enough
memory space, when there is only one cpu.

And thus causes the following failure:

  $ perf ftrace ls
  failed to reset ftrace
  $

This patch fixes the calculation of the cpumask string size.

Signed-off-by: He Zhe <zhe.he@windriver.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Fixes: dc23103278c5 ("perf ftrace: Add support for -a and -C option")
Link: http://lkml.kernel.org/r/1564734592-15624-1-git-send-email-zhe.he@windriver.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>