BUG/MINOR: activity: fix reporting of task latency

BUG/MINOR: activity: fix reporting of task latency

In 2.4, "show tasks" was introduced by commit 7eff06e162 ("MINOR:
activity: add a new "show tasks" command to list currently active tasks")
to expose some info about running tasks. The latency is not correct
because it's a u32 subtracted from a u64. It ought to have been casted
to u32 for the operation, which is what this patch does.

This can be backported to 2.4.