perf_counter: Full task tracing

perf_counter: Full task tracing

In order to be able to distinguish between no samples due to
inactivity and no samples due to task ended, Arjan asked for
PERF_EVENT_EXIT events. This is useful to the boot delay
instrumentation (bootchart) app.

This patch changes the PERF_EVENT_FORK to be emitted on every
clone, and adds PERF_EVENT_EXIT to be emitted on task exit,
after the task's counters have been closed.

This task tracing is controlled through: attr.comm || attr.mmap
and through the new attr.task field.

Suggested-by: Arjan van de Ven <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Anton Blanchard <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
[ cleaned up perf_counter.h a bit ]
Signed-off-by: Ingo Molnar <[email protected]>