osd/ReplicatedPG: mark_unrollbackable when _rollback_to head

We fell into the case in _rollback_to where we just set ctx->modify = true
and don't explicitly mark the ctx and unrollbackable.  Later, we screw up
in proc_replica_log as a result because we think we can rollback this
update to the head when in reality we cannot.

Fixes: #7907
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1579 from ceph/wip-7907

osd/ReplicatedPG: mark_unrollbackable when _rollback_to head

Reviewed-by: Samuel Just <sam.just@inktank.com>