perf machine: Fix the search for the kernel DSO on the unified list

When unifying the user_dsos and kernel_dsos a bug was introduced by
inverting the check for dso->kernel, fix it.

Fixes: 3d39ac538629 ("perf machine: No need to have two DSOs lists")
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/tip-xnrnq0kams3s2z9ek1wjb506@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>