scsi: iscsi: flush running unbind operations when removing a session

In some cases, the iscsi_remove_session() function is called while an
unbind_work operation is still running.  This may cause a situation where
sysfs objects are removed in an incorrect order, triggering a kernel
warning.

[  605.249442] ------------[ cut here ]------------
[  605.259180] sysfs group 'power' not found for kobject 'target2:0:0'
[  605.321371] WARNING: CPU: 1 PID: 26794 at fs/sysfs/group.c:235 sysfs_remove_group+0x76/0x80
[  605.341266] Modules linked in: dm_service_time target_core_user target_core_pscsi target_core_file target_core_iblock iscsi_target_mod target_core_mod nls_utf8 isofs ppdev bochs_drm nfit ttm libnvdimm drm_kms_helper syscopyarea sysfillrect sysimgblt joydev pcspkr fb_sys_fops drm i2c_piix4 sg parport_pc parport xfs libcrc32c dm_multipath sr_mod sd_mod cdrom ata_generic 8021q garp mrp ata_piix stp crct10dif_pclmul crc32_pclmul llc libata crc32c_intel virtio_net net_failover ghash_clmulni_intel serio_raw failover sunrpc dm_mirror dm_region_hash dm_log dm_mod be2iscsi bnx2i cnic uio cxgb4i cxgb4 libcxgbi libcxgb qla4xxx iscsi_boot_sysfs iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi
[  605.627479] CPU: 1 PID: 26794 Comm: kworker/u32:2 Not tainted 4.18.0-60.el8.x86_64 #1
[  605.721401] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20180724_192412-buildhw-07.phx2.fedoraproject.org-1.fc29 04/01/2014
[  605.823651] Workqueue: scsi_wq_2 __iscsi_unbind_session [scsi_transport_iscsi]
[  605.830940] RIP: 0010:sysfs_remove_group+0x76/0x80
[  605.922907] Code: 48 89 df 5b 5d 41 5c e9 38 c4 ff ff 48 89 df e8 e0 bf ff ff eb cb 49 8b 14 24 48 8b 75 00 48 c7 c7 38 73 cb a7 e8 24 77 d7 ff <0f> 0b 5b 5d 41 5c c3 0f 1f 00 0f 1f 44 00 00 41 56 41 55 41 54 55
[  606.122304] RSP: 0018:ffffbadcc8d1bda8 EFLAGS: 00010286
[  606.218492] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[  606.326381] RDX: ffff98bdfe85eb40 RSI: ffff98bdfe856818 RDI: ffff98bdfe856818
[  606.514498] RBP: ffffffffa7ab73e0 R08: 0000000000000268 R09: 0000000000000007
[  606.529469] R10: 0000000000000000 R11: ffffffffa860d9ad R12: ffff98bdf978e838
[  606.630535] R13: ffff98bdc2cd4010 R14: ffff98bdc2cd3ff0 R15: ffff98bdc2cd4000
[  606.824707] FS:  0000000000000000(0000) GS:ffff98bdfe840000(0000) knlGS:0000000000000000
[  607.018333] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  607.117844] CR2: 00007f84b78ac024 CR3: 000000002c00a003 CR4: 00000000003606e0
[  607.117844] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  607.420926] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  607.524236] Call Trace:
[  607.530591]  device_del+0x56/0x350
[  607.624393]  ? ata_tlink_match+0x30/0x30 [libata]
[  607.727805]  ? attribute_container_device_trigger+0xb4/0xf0
[  607.829911]  scsi_target_reap_ref_release+0x39/0x50
[  607.928572]  scsi_remove_target+0x1a2/0x1d0
[  608.017350]  __iscsi_unbind_session+0xb3/0x160 [scsi_transport_iscsi]
[  608.117435]  process_one_work+0x1a7/0x360
[  608.132917]  worker_thread+0x30/0x390
[  608.222900]  ? pwq_unbound_release_workfn+0xd0/0xd0
[  608.323989]  kthread+0x112/0x130
[  608.418318]  ? kthread_bind+0x30/0x30
[  608.513821]  ret_from_fork+0x35/0x40
[  608.613909] ---[ end trace 0b98c310c8a6138c ]---

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Acked-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>