tools/rados: always call rados.shutdown() before exit()

When connected to ceph cluster, if call exit(1) directly, will cause the finisher
thread segmentation fault as follows,

Caught signal (Segmentation fault) **in thread 7f45377fe700 thread_name:fn_anonymous

run 'rados mapext' without other arguments could easily get it. This patch fixes it
by calling rados.shutdown() before exit()

Fixes: http://tracker.ceph.com/issues/36732

Signed-off-by: Li Wang <[email protected]>