perf: Optimize perf_output_begin()

There's no point in re-doing the memory-barrier when we fail the
cmpxchg(). Also placing it after the space reservation loop makes it
clearer it only separates the userpage->tail read from the data
stores.

Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Michael Neuling <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: [email protected]
Cc: Vince Weaver <[email protected]>
Cc: Victor Kaplansky <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Anton Blanchard <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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