perf callchain: Use 'struct map_symbol' in 'struct callchain_cursor_node'

To ease passing around map+symbol, just like done for other parts of the
tree recently.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>