rgw/rados: get_part_obj_state() fixes accounted_size when uncompressed

rgw/rados: get_part_obj_state() fixes accounted_size when uncompressed

the part head objects don't have a RGW_ATTR_MANIFEST attribute, so
get_obj_state_impl() isn't able to set the correct
RGWObjState::accounted_size unless RGW_ATTR_COMPRESSION provides one

get_part_obj_state() builds a fake manifest in memory to represent the
part and updates state.size accordingly, but it hadn't corrected the
value of state.accounted_size

Signed-off-by: Casey Bodley <cbodley@redhat.com>