Log Message:
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simplify cleanup in storage_copy
struct lxc_rootfs has an lxc_storage pointer, and vice versa.
put_lxc_rootfs(r) will free the storage it points to, but
freeing the storage won't free the lxc_rootfs.
By setting new_rootfs->storage and calling put_lxc_rootfs(new_rootfs),
which we weren't doing, we can avoid having to manually close the
dfd_idmapped.