commit b3743c71 upstream.

For all the entries allocated from the ring cmd area, the memory is
something like the stack memory, which will always reserve the old
data, so the entry->req.iov_bidi_cnt maybe none zero.

On some environments, the crash could be reproduce very easy and some
not. The following is the crash core trace as reported by Damien:

[  240.143969] CPU: 0 PID: 1285 Comm: iscsi_trx Not tainted 4.12.0-rc1+ #3
[  240.150607] Hardware name: ASUS All Series/H87-PRO, BIOS 2104 10/28/2014
[  240.157331] task: ffff8807de4f5800 task.stack: ffffc900047dc000
[  240.163270] RIP: 0010:memcpy_erms+0x6/0x10
[  240.167377] RSP: 0018:ffffc900047dfc68 EFLAGS: 00010202
[  240.172621] RAX: ffffc9065db85540 RBX: ffff8807f7980000 RCX: 0000000000000010
[  240.179771] RDX: 0000000000000010 RSI: ffff8807de574fe0 RDI: ffffc9065db85540
[  240.186930] RBP: ffffc900047dfd30 R08: ffff8807de41b000 R09: 0000000000000000
[  240.194088] R10: 0000000000000040 R11: ffff8807e9b726f0 R12: 00000006565726b0
[  240.201246] R13: ffffc90007612ea0 R14: 000000065657d540 R15: 0000000000000000
[  240.208397] FS:  0000000000000000(0000) GS:ffff88081fa00000(0000) knlGS:0000000000000000
[  240.216510] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  240.222280] CR2: ffffc9065db85540 CR3: 0000000001c0f000 CR4: 00000000001406f0
[  240.229430] Call Trace:
[  240.231887]  ? tcmu_queue_cmd+0x83c/0xa80
[  240.235916]  ? target_check_reservation+0xcd/0x6f0
[  240.240725]  __target_execute_cmd+0x27/0xa0
[  240.244918]  target_execute_cmd+0x232/0x2c0
[  240.249124]  ? __local_bh_enable_ip+0x64/0xa0
[  240.253499]  iscsit_execute_cmd+0x20d/0x270
[  240.257693]  iscsit_sequence_cmd+0x110/0x190
[  240.261985]  iscsit_get_rx_pdu+0x360/0xc80
[  240.267565]  ? iscsi_target_rx_thread+0x54/0xd0
[  240.273571]  iscsi_target_rx_thread+0x9a/0xd0
[  240.279413]  kthread+0x113/0x150
[  240.284120]  ? iscsi_target_tx_thread+0x1e0/0x1e0
[  240.290297]  ? kthread_create_on_node+0x40/0x40
[  240.296297]  ret_from_fork+0x2e/0x40
[  240.301332] Code: 90 90 90 90 90 eb 1e 0f 1f 00 48 89 f8 48 89 d1 48
c1 e9 03 83 e2 07 f3 48 a5 89 d1 f3 a4 c3 66 0f 1f 44 00 00 48 89 f8 48
89 d1 <f3> a4 c3 0f 1f 80 00 00 00 00 48 89 f8 48 83 fa 20 72 7e 40 38
[  240.321751] RIP: memcpy_erms+0x6/0x10 RSP: ffffc900047dfc68
[  240.328838] CR2: ffffc9065db85540
[  240.333667] ---[ end trace b7e5354cfb54d08b ]---

To fix this, just memset all the entry memory before using it, and
also to be more readable we adjust the bidi code.

Fixed: fe25cc34(tcmu: Recalculate the tcmu_cmd size to save cmd area
		memories)
Reported-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Tested-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Reported-by: Damien Le Moal <damien.lemoal@wdc.com>
Tested-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Cc: <stable@vger.kernel.org> # 4.12+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>