MINOR: task: permanently enable latency measurement on tasklets

MINOR: task: permanently enable latency measurement on tasklets

When tasklet latency measurement was enabled in 2.4 with commit b2285de04
("MINOR: tasks: also compute the tasklet latency when DEBUG_TASK is set"),
the feature was conditionned on DEBUG_TASK because the field would add 8
bytes to the struct tasklet.

This approach was not a very good idea because the struct ends on an int
anyway thus it does finish with a 32-bit hole regardless of the presence
of this field. What is true however is that adding it turned a 64-byte
struct to 72-byte when caller debugging is enabled.

This patch revisits this with a minor change. Now only the lowest 32
bits of the call date are stored, so they always fit in the remaining
hole, and this allows to remove the dependency on DEBUG_TASK. With
debugging off, we're now seeing a 48-byte struct, and with debugging
on it's exactly 64 bytes, thus still exactly one cache line. 32 bits
allow a latency of 4 seconds on a tasklet, which already indicates a
completely dead process, so there's no point storing the upper bits at
all. And even in the event it would happen once in a while, the lost
upper bits do not really add any value to the debug reports. Also, now
one tasklet wakeup every 4 billion will not be sampled due to the test
on the value itself. Similarly we just don't care, it's statistics and
the measurements are not 9-digit accurate anyway.