fs: Use this_cpu_xx operations in buffer.c

Optimize various per cpu area operations through these new percpu
operations.  These operations avoid address calculations through the
use of segment prefixes and multiple memory references through RMW
instructions etc.

Reduces code size:

Before:

christoph@linux-2.6$ size fs/buffer.o
   text	   data	    bss	    dec	    hex	filename
  19169	     80	     28	  19277	   4b4d	fs/buffer.o

After:

christoph@linux-2.6$ size fs/buffer.o
   text	   data	    bss	    dec	    hex	filename
  19138	     80	     28	  19246	   4b2e	fs/buffer.o

V3->V4:
	- Move the use of this_cpu_inc_return into a later patch so that
	  this one can go in without percpu infrastructure changes.

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>