PG::find_best_info: fix log_tail component

The previous logic should have kept the current best info if it found a
replica which best could log-recover, but p couldn't.  However, the
continue in that loop advanced the inner loop instead of the outer loop
allowing the primary case to take over in cases where best had a longer
tail.  Instead, we will prefer the longer tail regardless of the other
infos to simplify the logic.

Fixes: #7755
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1506 from ceph/wip-7755

PG::find_best_info: fix log_tail component

Reviewed-by: Sage Weil <sage@inktank.com>