osd/PrimaryLogPG.cc: limit trimming at can_rollback_to

osd/PrimaryLogPG.cc: limit trimming at can_rollback_to

This change is motivated by the failures seen in the multimds suite,
where we hit assert(s <= can_rollback_to), while trimming the log in ec
pools.

This is due to the fact that we had removed limits on the trim_to value to
address https://tracker.ceph.com/issues/23979.

But, seems that this could be dangerous for ec pools. So, keep the
can_rollback_to limit, while calculating the trim_to value.

Fixes: http://tracker.ceph.com/issues/21416
Signed-off-by: Neha Ojha <nojha@redhat.com>
(cherry picked from commit 4b5c6b88d444e2173e716fe4890717873c8dc8e5)