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: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>