execve() and compat_execve() instrumentation

As I am digging into LTTng's instrumentation, we actually put it right
after open_exec() and before sched_exec(). We use this event to assign
the process name to the pids, but it is also interesting to have the
causality link "exec" causing a "sched_migrate_task" (called from
sched_exec) in the right order. As this causality can be extracted from
the system call information, we don't miss much by changing the exec
instrumentation location so it sits within the if (retval >= 0) branch.
The good side is that we won't apply a false filename when a exec system
call fails due to a lack of free memory. I will change LTTng's
instrumentation accordingly in the next version.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Candidate, Ecole Polytechnique de Montreal
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