rgw: destroy get_obj handle in copy_obj()

Fixes: #6176
Backport: dumpling
We take different code paths in copy_obj, make sure we close the handle
when we exit the function. Move the call to finish_get_obj() out of
copy_obj_data() as we don't create the handle there, so that should
makes code less confusing and less prone to errors.
Also, note that RGWRados::get_obj() also calls finish_get_obj(). For
everything to work in concert we need to pass a pointer to the handle
and not the handle itself. Therefore we needed to also change the call
to copy_obj_data().

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>