perf maps: Purge all maps from the 'names' tree

Add function __maps__purge_names() to purge all maps from the names
tree.  We need to cleanup the names tree in maps__exit().

Detected with gcc's ASan.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Eric Saint-Etienne <eric.saint.etienne@oracle.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Fixes: 1e628569 ("perf symbols: Fix slowness due to -ffunction-section")
Link: http://lkml.kernel.org/r/20190316080556.3075-12-changbin.du@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>