f2fs: fix the fault of calculating blkstart twice

When the zone type is BLK_ZONE_TYPE_CONVENTIONAL, the blkstart is
calculated twice.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>