Commit ca6c41c5 sets the ppid based on what is read from the
/proc/pid/status file when synthesizing fork events.

This is correct thing to do for new processes but not threads of a
process.

Fix ppid for threads to be the main thread when synthesizing fork events
(ie., assume main thread spawned all sub-threads in a process).
Reported-by: NArnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Signed-off-by: NDavid Ahern <david.ahern@oracle.com>
Tested-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: NDon Zickus <dzickus@redhat.com>
Link: http://lkml.kernel.org/r/1428598107-178999-1-git-send-email-david.ahern@oracle.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>