Create update_shadow_stats() which is then used in both read_counter_aggr()
and read_counter().

This not only simplifies the code but also fixes a bug: HW_CACHE_REFERENCES
was not updated in read_counter().
Acked-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/n/tip-9uc55z3g88r47exde7zxjm6p@git.kernel.orgSigned-off-by: NIngo Molnar <mingo@elte.hu>