scsi: sg: sg_write(): __get_user() can fail...

Link: https://lore.kernel.org/r/20191017193925.25539-3-viro@ZenIV.linux.org.uk


Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>