[BLOCK] increase size of disk stat counters

The kernel's representation of the disk statistics uses the type unsigned
which is 32b on both 32b and 64b platforms.  Unfortunately, most system
tools that work with these numbers that are exported in /proc/diskstats
including iostat read these numbers into unsigned longs.  This works fine
on 32b platforms and when the number of IO transactions are small on 64b
platforms.  However, when the numbers wrap on 64b platforms & you read the
numbers into unsigned longs, and compare the numbers to previous readings,
then you get an unsigned representation of a negative number.  This looks
like a very large 64b number & gives you bizarre readouts in iostat:

ilc4: Device:    rrqm/s wrqm/s r/s    w/s  rsec/s  wsec/s    rkB/s wkB/s avgrq-sz avgqu-sz   await  svctm  %util
ilc4: sda        5.50   0.00   143.96 0.00 307496983987862656.00 0.00 153748491993931328.00     0.00 2136028725038430.00     7.94   55.12    5.59  80.42

Though fixing iostat in user space is possible, and a quick survey
indicates that several other similar tools also use unsigned longs when
processing /proc/diskstats.  Therefore, it seems like a better approach
would be to extend the length of the disk_stats structure on 64b
architectures to 64b.  The following patch does that.  It should not affect
the operation on 32b platforms.

Signed-off-by: Ben Woodard <woodard@redhat.com>
Cc: Rick Lindsley <ricklind@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Jens Axboe <axboe@suse.de>