scsi: scsi_debug: Fix memory leak on module unload

Since commit 80c49563 ("scsi: scsi_debug: implement IMMED bit") there
are long delays in F_SYNC_DELAY and F_SSU_DELAY.  This can cause a memory
leak in schedule_resp(), which can be invoked while unloading the
scsi_debug module: free_all_queued() had already freed all sd_dp and
schedule_resp will alloc a new one, which will never get freed.  Here's the
kmemleak report while running xfstests generic/350:

unreferenced object 0xffff88007d752b00 (size 128):
  comm "rmmod", pid 26940, jiffies 4295816945 (age 7.588s)
  hex dump (first 32 bytes):
    00 2b 75 7d 00 88 ff ff 00 00 00 00 00 00 00 00  .+u}............
    00 00 00 00 00 00 00 00 8e 31 a2 34 5f 03 00 00  .........1.4_...
  backtrace:
    [<000000002abd83d0>] 0xffffffffa000705e
    [<000000004c063fda>] scsi_dispatch_cmd+0xc7/0x1a0
    [<000000000c119a00>] scsi_request_fn+0x251/0x550
    [<000000009de0c736>] __blk_run_queue+0x3f/0x60
    [<000000001c4453c8>] blk_execute_rq_nowait+0x98/0xd0
    [<00000000d17ec79f>] blk_execute_rq+0x3a/0x50
    [<00000000a7654b6e>] scsi_execute+0x113/0x250
    [<00000000fd78f7cd>] sd_sync_cache+0x95/0x160
    [<0000000024dacb14>] sd_shutdown+0x9b/0xd0
    [<00000000e9101710>] sd_remove+0x5f/0xb0
    [<00000000c43f0d63>] device_release_driver_internal+0x13c/0x1f0
    [<00000000e8ad57b6>] bus_remove_device+0xe9/0x160
    [<00000000713a7b8a>] device_del+0x120/0x320
    [<00000000e5db670c>] __scsi_remove_device+0x115/0x150
    [<00000000eccbef30>] scsi_forget_host+0x20/0x60
    [<00000000cd5a0738>] scsi_remove_host+0x6d/0x120

Cc: stable@vger.kernel.org # v4.17+
Signed-off-by: Luis Henriques <lhenriques@suse.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>