this_cpu: Use this_cpu operations for NFS statistics

Simplify NFS statistics and allow the use of optimized
arch instructions.

Acked-by: Tejun Heo <tj@kernel.org>
CC: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Tejun Heo <tj@kernel.org>