perf report: Update for the new FORK/EXIT events

Since FORK is now also issued for threads, detect those by
comparing the parent and child PID.

Teach it about EXIT events and ignore them.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>