Some usb disks need an extra opportunity to divulge their capacity (e.g. M-Systems/SanDisk 1GB flash drive)

Reference: /n/sources/patch/applied/usb-disk-capacity
Date: Wed Jul 27 17:46:50 CES 2011
Signed-off-by: miller@hamnavoe.com

--- /sys/src/cmd/usb/disk/disk.c	Wed Jul 27 17:45:42 2011
+++ /sys/src/cmd/usb/disk/disk.c	Wed Jul 27 17:45:39 2011
@@ -481,6 +481,8 @@
 		count = usbdirread(fs, q, data, count, offset, dirgen, nil);
 		break;
 	case Qctl:
+		if(lun->lbsize <= 0)
+			umscapacity(lun);
 		s = buf;
 		e = buf + sizeof(buf);
 		if(lun->flags & Finqok)