Fix: Sleeping function called from invalid context

It affects system call instrumentation for accept, accept4 and connect,
only on the x86-64 architecture.

We need to use the LTTng accessing functions to touch user-space memory,
which take care of disabling the page fault handler, so we don't preempt
while in preempt-off context (tracepoints disable preemption).

Fixes #1111

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>