Keep data_head up-to-date irrespective of notifications. This fixes
the case where you disable a counter and don't get a notification for
the last few pending events, and it also allows polling usage.

[ Impact: increase precision of perfcounter mmap-ed fields ]
Suggested-by: NCorey Ashford <cjashfor@linux.vnet.ibm.com>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <20090505155436.925084300@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>