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.

(cherry picked from commit 17d33923484ba5661f8938b55cc825b156766955)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 38233771977e142264a76b51c5d56068180d5fa2)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 885d50f802aa9b8c79eba8db1925cdb1ed4c3a01)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>