posixdriver: fix cksum_type, flags propagation

posixdriver: fix cksum_type, flags propagation

Posixdriver doesn't serialize POSIXMultipartUpload, but rather a
member mp_obj of type POSIXMPObj--so to avoid losing the latter's
inherited cksum_type and cksum_flags members (which are already
copied in), copy them out in POSIXMultiPartUpload::get_info() which
we need to call to copy out dest_placement anyway.

(oops, chksum_type was copied in, but not cksum_flags)

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>