osd/PG: do not blindly roll forward to log.head

osd/PG: do not blindly roll forward to log.head

If we are told we can roll forward by the primary, we should only roll
forward as far as the primary says we can.

This probably came out of the similar case in append_log(), but notably
that roll_forward() only happens if !transaction_applied (i.e., backfill
target), and that condition is not checked here.

Fixes: http://tracker.ceph.com/issues/24597
Signed-off-by: Sage Weil <sage@redhat.com>