scsi: sg: allocate with __GFP_ZERO in sg_build_indirect()

commit a45b599ad808c3c982fdcdc12b0b8611c2f92824 upstream.

This shall help avoid copying uninitialized memory to the userspace when
calling ioctl(fd, SG_IO) with an empty command.

Reported-by:  <syzbot+7d26fc1eea198488deab@syzkaller.appspotmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alexander Potapenko <glider@google.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>