perf tools: Add default handler for mmap2 events

Commands that do not implement an mmap2 handler should at least not die
with a segfault when processing files with MMAP2 events.

Signed-off-by: David Ahern <dsahern@gmail.com>
Link: http://lkml.kernel.org/r/1379900700-5186-5-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>