fs: Use this_cpu_inc_return in buffer.c

__this_cpu_inc can create a single instruction with the same effect
as the _get_cpu_var(..)++ construct in buffer.c.

Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>