ReplicatedPG: consistently use ctx->at_version.version for stashed object

Otherwise, two ops might end up using the same version number.

Fixes: #7632
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1392 from ceph/wip-7632

ReplicatedPG: consistently use ctx->at_version.version for stashed objec...

Reviewed-by: Sage Weil <sage@inktank.com>