perf unwind: Report error from dwfl_attach_state

In verbose level 2, errors returned by libdw are reported in most cases,
but not when calling dwfl_attach_state.

Since elfutils v 0.160 (2014), dwfl_attach_state sets the error code to
report failure cause. On failure, log the reported error.
Signed-off-by: Martin Vuille <jpmv27@aim.com>
Reviewed-by: Kim Phillips <kim.phillips@arm.com>
Link: http://lkml.kernel.org/r/20180318175053.4222-1-jpmv27@aim.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/unwind-libdw.c

@@ -236,7 +236,8 @@ int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 	if (err)
 		goto out;
 
-	if (!dwfl_attach_state(ui->dwfl, EM_NONE, thread->tid, &callbacks, ui))
+	err = !dwfl_attach_state(ui->dwfl, EM_NONE, thread->tid, &callbacks, ui);
+	if (err)
 		goto out;
 
 	err = dwfl_getthread_frames(ui->dwfl, thread->tid, frame_callback, ui);