The per-cpu disk stats are being updated in a non-preempt-safe manner in a
couple of places.

The patch introduces introduces preempt and non-preempt versions of the
statistics code and updates the block code to use the appropriate ones.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>