perf cpumap: Fix snprintf overflow check

'snprintf' returns the number of characters which would be generated for
the given input.

If the returned value is *greater than* or equal to the buffer size, it
means that the output has been truncated.

Fix the overflow test accordingly.

Fixes: 7780c25b ("perf tools: Allow ability to map cpus to nodes easily")
Fixes: 92a7e127 ("perf cpumap: Add cpu__max_present_cpu()")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Suggested-by: David Laight <David.Laight@ACULAB.COM>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: He Zhe <zhe.he@windriver.com>
Cc: Jan Stancek <jstancek@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kernel-janitors@vger.kernel.org
Link: http://lore.kernel.org/lkml/20200324070319.10901-1-christophe.jaillet@wanadoo.fr


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