cls_log: always return final marker from log_list

cls_log: always return final marker from log_list

There's no reason to restrict returning the marker to the case where
less than the whole log is returned, since there's already a truncated
flag to tell the client what happened.

Giving the client the last marker makes it easy to consume when the
log entries do not contain their own marker. If the last marker is not
returned, the client cannot get the last marker without racing with
updates to the log.

Backport: dumpling
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>