I was assuming rb_erase() was setting things up like list_del_init, but
the fact that thread__delete() was being sucessfull is because the last
thing before deleting is to remove the thread from the
machine->dead_threads list, using list_del_init(), that has the same
effect as using rb_erase_init()...

Introduce this function so that we can use it when removing objects from
rb_trees.

Then we will be able to BUG_ON(still on a list) in destructors.

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: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-55b16mbtndjyd7zzg8nmnamx@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>