ECBackend: when removing the temp obj, use the right shard

Introduced in d0b1094ff7b98ef9262ecb45ee8324853003a77c
Fixes: #7681
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1436 from ceph/wip-7681

ECBackend: when removing the temp obj, use the right shard

Reviewed-by: Sage Weil <sage@inktank.com>