perf machine: Add missing dsos->root rbtree root initialization

A segfault happens on 'perf test hists_link' because we end up using a
struct machines on the stack, and then machines__init() was not
initializing the newly introduced rb_root, just the existing list_head.

When we introduced struct dsos, to group the two ways to store dsos,
i.e. the linked list and the rbtree, we didn't turned the initialization
done in:

	machines__init(machines->host) ->
		machine__init() ->
			INIT_LIST_HEAD

into a dsos__init() to keep on initializing the list_head but _as well_
initializing the rb_root, oops.

All worked because outside perf-test we probably zalloc the whole thing
which ends up initializing it in to NULL.

So the problem looks contained to 'perf test' that uses it on stack,
etc.
Reported-by: NJiri Olsa <jolsa@redhat.com>
Acked-by: Waiman Long <Waiman.Long@hp.com>,
Cc: Adrian Hunter <adrian.hunter@intel.com>,
Cc: Don Zickus <dzickus@redhat.com>
Cc: Douglas Hatch <doug.hatch@hp.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hp.com>
Cc: Waiman Long <Waiman.Long@hp.com>,
Link: http://lkml.kernel.org/r/20141014180353.GF3198@kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/machine.c

@@ -13,12 +13,18 @@
 #include <symbol/kallsyms.h>
 #include "unwind.h"
 
+static void dsos__init(struct dsos *dsos)
+{
+	INIT_LIST_HEAD(&dsos->head);
+	dsos->root = RB_ROOT;
+}
+
 int machine__init(struct machine *machine, const char *root_dir, pid_t pid)
 {
 	map_groups__init(&machine->kmaps);
 	RB_CLEAR_NODE(&machine->rb_node);
-	INIT_LIST_HEAD(&machine->user_dsos.head);
-	INIT_LIST_HEAD(&machine->kernel_dsos.head);
+	dsos__init(&machine->user_dsos);
+	dsos__init(&machine->kernel_dsos);
 
 	machine->threads = RB_ROOT;
 	INIT_LIST_HEAD(&machine->dead_threads);