perf_counter: x86: Set the period in the intel overflow handler

Commit 9e350de37ac960 ("perf_counter: Accurate period data")
missed a spot, which caused all Intel-PMU samples to have a
period of 0.

This broke auto-freq sampling.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>