Continue scanning a SCSI bus when a LUN is reported not present

This fixes disk attachment under Qemu when there is no disk on LUN 0 on
a SCSI bus but there is a disk on LUN 1. The inquiry for LUN 0 returns
SID_QUAL_LU_NOTPRESENT & T_NODEVICE.  Quirks are only checked if neither
one of those are set, so cannot use a quirk entry.

Use case 1: Proxmox 6 configures each disk on its own bus when using
the "Virtio SCSI single" SCSI controller. However, while the "scsi0"
disk is on LUN 0, the "scsi1" disk is on LUN 1.

Use case 2: A Linode boot profile with multiple disks results in
the first disk ("sda") on LUN 1, while the second disk ("sdb") is
on LUN 0, each on their own bus.

Index: scsiconf.c
===================================================================
RCS file: /cvsroot/src/sys/dev/scsipi/scsiconf.c,v
retrieving revision 1.287
diff -u -p -r1.287 scsiconf.c
--- scsiconf.c	2 May 2020 06:45:53 -0000	1.287
+++ scsiconf.c	11 Jul 2020 11:24:57 -0000
@@ -883,6 +883,8 @@ scsi_probe_device(struct scsibus_softc *
 		break;
 
 	case SID_QUAL_LU_NOTPRESENT:
+		docontinue = 1;
+		/* FALLTHROUGH */
 	case SID_QUAL_reserved:
 	case SID_QUAL_LU_NOT_SUPP:
 		goto bad;