rgw: don't copy object idtag when copying object

Fixes: #5204
When copying object we ended up also copying the original
object idtag which overrode the newly generated one. When
refcount put is called with the wrong idtag the count
does't go down.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
rgw: only append prefetched data if reading from head

Fixes: #5209
Backport: bobtail, cuttlefish
If the head object wrongfully contains data, but according to the
manifest we don't read from the head, we shouldn't copy the prefetched
data. Also fix the length calculation for that data.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>