Use preempt_enable() instead of preempt_enable_no_resched()

Hi,

LTTng 0.6.56 starts using the preemption reactivation which calls the scheduler
at the end of the instrumentation site. It has no effect at sites where the
preemption is already disactivated around the marker, so it should be safe in
every spots in sched.c. See my email to LKML for the discussion about the other
instrumentation sites.

Mathieu

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