PGLog::IndexedLog::trim(): rollback_info_trimmed_to_riter may be log.rend()

Fixes: #9731
Backport: giant, firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #2707 from ceph/wip-9731

PGLog::IndexedLog::trim(): rollback_info_trimmed_to_riter may be log.ren...

Reviewed-by: Sage Weil <sage@redhat.com>