genhd: overlapping variable definition

This fixes the sparse warning:
fs/ext4/super.c:2390:40: warning: symbol 'i' shadows an earlier one
fs/ext4/super.c:2368:22: originally declared here

Using 'i' in a macro is dubious practice.
Signed-off-by: NStephen Hemminger <shemminger@vyatta.com>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

include/linux/genhd.h

@@ -256,9 +256,9 @@ extern struct hd_struct *disk_map_sector_rcu(struct gendisk *disk,
 #define part_stat_read(part, field)					\
 ({									\
 	typeof((part)->dkstats->field) res = 0;				\
-	int i;								\
-	for_each_possible_cpu(i)					\
-		res += per_cpu_ptr((part)->dkstats, i)->field;		\
+	unsigned int _cpu;						\
+	for_each_possible_cpu(_cpu)					\
+		res += per_cpu_ptr((part)->dkstats, _cpu)->field;	\
 	res;								\
 })