rgw/posix: fix crash in radosgw-admin

The POSIXBucket copy constructor incorrectly calls .get() on a
on a temporary unique_ptr returned by clone(), causing immediate
deletion of the Directory object. This leaves a dangling pointer
that triggers a segfault during destruction.

Signed-off-by: Nithya Balachandran <nithya.balachandran@ibm.com>