scsi: target: core: Get rid of warning in compare_and_write_do_cmp()

Rename function local variable i to sg_cnt so we can get rid of the
shadow variable compilation warning:

	unsigned int i;
		     ^
	int i;
	^

Link: https://lore.kernel.org/r/20210228055645.22253-6-chaitanya.kulkarni@wdc.com


Reviewed-by: Mike Christie <michael.christie@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>