scsi: iscsi: Use system_unbound_wq for destroy_work

stable inclusion
from stable-v5.10.112
commit 22608545b834b855c0a7fd9f675edc884b7d21a3
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5HL0X

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=22608545b834b855c0a7fd9f675edc884b7d21a3

--------------------------------

[ Upstream commit b25b957d ]

Use the system_unbound_wq for async session destruction. We don't need a
dedicated workqueue for async session destruction because:

 1. perf does not seem to be an issue since we only allow 1 active work.

 2. it does not have deps with other system works and we can run them in
    parallel with each other.

Link: https://lore.kernel.org/r/20210525181821.7617-6-michael.christie@oracle.com


Reviewed-by: Lee Duncan <lduncan@suse.com>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>