perf target: Simplify handling of strerror_r return

To deal with forwarding the strerror_r (GNU) return we need to check if
the returned value is the buffer we passed or maybe some constant
(unknown error), simplify that action by using scnprintf, that will do
all the buflen size checks, trimming if needed.

Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-d0ik6i5gjew56j0qphql28ou@git.kernel.org


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