i915/gem_eio: Flush RCU before timing our own critical sections

We cannot control how long RCU takes to find a quiescent point as that
depends upon the background load and so may take an arbitrary time.
Instead, let's try to avoid that impacting our measurements by inserting
an rcu_barrier() before our critical timing sections and hope that hides
the issue, letting us always perform a fast reset. Fwiw, we do the
expedited RCU synchronize, but that is not always enough.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>