crimson/osd: fix assertion fail in prepare_clone

crimson/osd: fix assertion fail in prepare_clone

Ops like rbd.assert_snapc_seq can return -ERANGE, but we were preparing
clones in OpsExecuter constructor before guard evaluation.

On retry with sparse_copyup, this led to ceph_assert(!existed) in
prepare_clone() due to the pre-created clone OBC.

This adds precise OBC invalidation during rollback using single key
clear.

Fixes: https://tracker.ceph.com/issues/72705
Fixes: https://tracker.ceph.com/issues/72709
Fixes: https://tracker.ceph.com/issues/72041
Fixes: https://tracker.ceph.com/issues/72650
Signed-off-by: Kautilya Tripathi <kautilya.tripathi@ibm.com>