vhost/scsi: increase VHOST_SCSI_PREALLOC_PROT_SGLS to 2048
The current value of VHOST_SCSI_PREALLOC_PROT_SGLS is too small to
accommodate larger I/Os, e.g. 16-32 MiB, when the VIRTIO_SCSI_F_T10_PI
feature bit is negotiated and the backing store supports T10 PI.