usb: uas: fix a plug & unplug racing

When a uas disk is plugged into an external hub, uas_probe()
will be called by the hub thread to do the probe. It will
first create a SCSI host and then do the scan for this host.
During the scan, it will probe the LUN using SCSI INQUERY command
which will be packed in the URB and submitted to uas disk.

There might be a chance that this external hub with uas disk
attached is unplugged during the scan. In this case, uas driver
will fail to submit the URB (due to the NOTATTACHED state of uas
device) and try to put this SCSI command back to request queue
waiting for next chance to run.

In normal case, this cycle will terminate when hub thread gets
disconnection event and calls into uas_disconnect() accordingly.
But in this case, uas_disconnect() will not be called because
hub thread of external hub gets stuck waiting for the completion
of this SCSI command. A deadlock happened.

In this fix, uas will call scsi_scan_host() asynchronously to
avoid the blocking of hub thread.

Signed-off-by: EJ Hsu <ejh@nvidia.com>
Acked-by: Oliver Neukum <oneukum@suse.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200130092506.102760-1-ejh@nvidia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>