vmstat: use raw_cpu_ops to avoid false positives on preemption checks

vmstat: use raw_cpu_ops to avoid false positives on preemption checks

vm counters are allowed to be racy.  Use raw_cpu_ops to avoid the
local_irq_disable overhead and to avoid preemption checks which will be
added to the __this_cpu operations.

[[email protected]: Add comment.  Again.]
Signed-off-by: Christoph Lameter <[email protected]>
Reported-by: Sergey Senozhatsky <[email protected]>
Cc: Dave Chinner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>