scsi: scsi_debug: Fix create target debugfs failure

Target debugfs entry is removed via async_schedule() which isn't drained
when adding same name target, so failure of "Directory 'target11:0:0' with
parent 'scsi_debug' already present!" can be triggered easily.

Fix it by switching to domain async schedule, and draining it before
adding new target debugfs entry.

Cc: Wenchao Hao <haowenchao2@huawei.com>
Fixes: f084fe52

 ("scsi: scsi_debug: Add debugfs interface to fail target reset")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Acked-by: Wenchao Hao <haowenchao22@gmail.com>
Link: https://lore.kernel.org/r/20240619013803.3008857-1-ming.lei@redhat.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>