x86, UV: Shorten access to BAU statistics structure

Use a pointer from the per-cpu BAU control structure to the
per-cpu BAU statistics structure.
We nearly always know the first before needing the second.

Signed-off-by: Cliff Wickman <cpw@sgi.com>
Cc: gregkh@suse.de
LKML-Reference: <E1OJvNy-0004aB-2k@eag09.americas.sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>