Revert "sd: remove __data_len hack for WRITE SAME"

This patch reverts commit f80de881 and avoids that sending a
WRITE SAME command to the iSCSI initiator triggers the following:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000014
TARGET_CORE[iSCSI]: Expected Transfer Length: 260096 does not match SCSI CDB Length: 512 for SAM Opcode: 0x41
IP: iscsi_tcp_segment_done+0x20b/0x310 [libiscsi_tcp]

Oops: 0000 [#1] SMP
Modules linked in: target_core_user uio target_core_iblock target_core_file iscsi_target_mod target_core_mod netconsole configfs crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel aes_x86_64 crypto_simd cryptd glue_helper virtio_console virtio_rng virtio_balloon serio_raw i2c_piix4 acpi_cpufreq button iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi ext4 jbd2 mbcache virtio_blk virtio_net psmouse floppy drm_kms_helper syscopyarea
sysfillrect sysimgblt fb_sys_fops ttm drm virtio_pci
CPU: 2 PID: 5 Comm: kworker/u8:0 Not tainted 4.10.0-rc5-debug+ #3
Workqueue: iscsi_q_0 iscsi_xmitworker [libiscsi]
RIP: 0010:iscsi_tcp_segment_done+0x20b/0x310 [libiscsi_tcp]
Call Trace:
 iscsi_sw_tcp_xmit_segment+0x84/0x120 [iscsi_tcp]
 iscsi_sw_tcp_pdu_xmit+0x51/0x180 [iscsi_tcp]
 iscsi_tcp_task_xmit+0xb3/0x290 [libiscsi_tcp]
 iscsi_xmit_task+0x4e/0xc0 [libiscsi]
 iscsi_xmitworker+0x243/0x330 [libiscsi]
 process_one_work+0x1d8/0x4b0
 worker_thread+0x49/0x4a0
 kthread+0x102/0x140

Fixes: f80de881

 ("sd: remove __data_len hack for WRITE SAME")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Jens Axboe <axboe@fb.com>
Cc: Lee Duncan <lduncan@suse.com>
Cc: Chris Leech <cleech@redhat.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@fb.com>