perf tools: cleanup initialization of attr->size

The perf_event_attr size needs to be initialized in all cases because it
captures the ABI version.

This patch moves the initialization of the field from the
perf_event_open() syscall stub to its proper location in the
event_attr_init().

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20120209151238.GA10272@quad
Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>