Fix: preemptible and migratable context error handling

When built against preempt-rt and preempt kernels, the "return 0" case
means success, but lttng-modules incorrectly prints an error in the
kernel log.

Given that we handle the -ENOSYS error in lttng_context_init, there is
no need to keep the ifdefs in that function.

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