UAS: fix deadlock in error handling and PM flushing work

UAS: fix deadlock in error handling and PM flushing work

A SCSI error handler and block runtime PM must not allocate
memory with GFP_KERNEL. Furthermore they must not wait for
tasks allocating memory with GFP_KERNEL.
That means that they cannot share a workqueue with arbitrary tasks.

Fix this for UAS using a private workqueue.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Fixes: f9dc024a2da1f ("uas: pre_reset and suspend: Fix a few races")
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200415141750.811-2-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>