scsi: target/iblock: fix WRITE SAME zeroing

SBC4 specifies that WRITE SAME requests with the UNMAP bit set to zero
"shall perform the specified write operation to each LBA specified by the
command".  Commit 2237498f ("target/iblock: Convert WRITE_SAME to
blkdev_issue_zeroout") modified the iblock backend to call
blkdev_issue_zeroout() when handling WRITE SAME requests with UNMAP=0 and a
zero data segment.

The iblock blkdev_issue_zeroout() call incorrectly provides a flags
parameter of 0 (bool false), instead of BLKDEV_ZERO_NOUNMAP.  The bool
false parameter reflects the blkdev_issue_zeroout() API prior to commit
ee472d83 ("block: add a flags argument to (__)blkdev_issue_zeroout")
which was merged shortly before 2237498f.

Link: https://lore.kernel.org/r/20200419163109.11689-1-ddiss@suse.de
Fixes: 2237498f

 ("target/iblock: Convert WRITE_SAME to blkdev_issue_zeroout")
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>