Currently cancellation is logged in get_next_task, but the function is
called by tablets code as well where we do not act upon its result, only
yield to the topology coordinator. But the topology coordinator will not
necessary do the cancellation as well since it can be busy with tablets
migration. As a result cancellation is logged, but not done which is
confusing. Fix it by logging cancellation when it is actually happens.
Fixes https://scylladb.atlassian.net/browse/SCYLLADB-1409Closesscylladb/scylladb#29471
ca24dd4a5f