perf_counter: more elaborate write API

Provide a begin, copy, end interface to the output buffer.

begin() reserves the space,
 copy() copies the data over, considering page boundaries,
  end() finalizes the event and does the wakeup.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Orig-LKML-Reference: <20090325113316.740550870@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>