perf_counter: fix update_userpage()

It just occured to me it is possible to have multiple contending
updates of the userpage (mmap information vs overflow vs counter).
This would break the seqlock logic.

It appear the arch code uses this from NMI context, so we cannot
possibly serialize its use, therefore separate the data_head update
from it and let it return to its original use.

The arch code needs to make sure there are no contending callers by
disabling the counter before using it -- powerpc appears to do this
nicely.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Paul Mackerras <paulus@samba.org>
Orig-LKML-Reference: <20090330171023.241410660@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>