Conversion of nr_bounce to a per zone counter

nr_bounce is only used for proc output.  So it could be left as an event
counter.  However, the event counters may not be accurate and nr_bounce is
categorizing types of pages in a zone.  So we really need this to also be a
per zone counter.

[akpm@osdl.org: bugfix]
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>