[driver] Wait for the correct disk to show up, rather than any disk

check_system_disk() neglects to set the status to a non-success value
before checking the disk signature, with the result that it will
return success as soon as fetch_partition_info() succeeds for any
disk.  (This is incorrect behaviour, but will have observable effects
only in a multi-disk setup where the SAN disks come online at
different times.)

Fix by setting status to STATUS_UNSUCCESSFUL before checking the disk
signature.

Signed-off-by: Michael Brown <mbrown@fensystems.co.uk>

src/driver/sanbootconf.c

@@ -292,6 +292,9 @@ static NTSTATUS check_system_disk ( PUNICODE_STRING name,
 	status = fetch_partition_info ( name, device, file, &info );
 	if ( ! NT_SUCCESS ( status ) )
 		goto err_fetch_partition_info;
+
+	/* Check for a matching disk signature */
+	status = STATUS_UNSUCCESSFUL;
 	switch ( info.PartitionStyle ) {
 	case PARTITION_STYLE_MBR:
 		DbgPrint ( "  MBR %08lx: \"%wZ\"\n",