perf machine: Don't open code assign dso->short_name

perf machine: Don't open code assign dso->short_name

Use dso__set_short_name instead, as it will release any previously,
possibly allocated, short name.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-1v39elw7v6nxczpntpp7ljwr@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>