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>