uas: add UNLINK_DATA_URBS flag

uas_unlink_data_urbs uses this to make sure the the scsi command is
not released while looking at it.  This will be needed when we start
calling uas_unlink_data_urbs in the request cancel code paths.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>