Sometimes we want to sort an existing rbtree by a different key,
introduce a template for that, that needs only to be provided the
rbtree root and the number of entries in it.

To do that a new rbtree will be created with extra space for each entry,
where possibly pre-calculated keys will be stored to be used in the
resort process and also later, when using the newly sorted rbtree.

Please check the following two changesets to see it in use for resorting
stats for threads and its syscalls in 'perf trace --summary'.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-9l6e1q34lmf3wwdeewstyakg@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>