scsi-generic: Remove bogus double complete

scsi-generic scsi_read_complete() should not -both- call the client
complete callback with SCSI_REASON_DATA -and- call
scsi_command_complete().  The former will cause the client to queue a
new read or write request, while the later will free the request data
structure, thus causing the new read or write request to use a
freed/stale structure when it completes.

This patch fixes the bug, fixing a crash with scsi-generic & RHEL5.5
installer.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>

hw/scsi-generic.c

@@ -172,9 +172,11 @@ static void scsi_read_complete(void * opaque, int ret)
     DPRINTF("Data ready tag=0x%x len=%d\n", r->req.tag, len);
 
     r->len = -1;
-    r->req.bus->complete(r->req.bus, SCSI_REASON_DATA, r->req.tag, len);
-    if (len == 0)
+    if (len == 0) {
         scsi_command_complete(r, 0);
+    } else {
+        r->req.bus->complete(r->req.bus, SCSI_REASON_DATA, r->req.tag, len);
+    }
 }
 
 /* Read more data from scsi device into buffer.  */