xen-blkfront: fix kernel panic with negotiate_mq error path

info->nr_rings isn't adjusted in case of ENOMEM error from
negotiate_mq(). This leads to kernel panic in error path.

Typical call stack involving panic -
 #8 page_fault at ffffffff8175936f
    [exception RIP: blkif_free_ring+33]
    RIP: ffffffffa0149491  RSP: ffff8804f7673c08  RFLAGS: 00010292
 ...
 #9 blkif_free at ffffffffa0149aaa [xen_blkfront]
 #10 talk_to_blkback at ffffffffa014c8cd [xen_blkfront]
 #11 blkback_changed at ffffffffa014ea8b [xen_blkfront]
 #12 xenbus_otherend_changed at ffffffff81424670
 #13 backend_changed at ffffffff81426dc3
 #14 xenwatch_thread at ffffffff81422f29
 #15 kthread at ffffffff810abe6a
 #16 ret_from_fork at ffffffff81754078

Cc: stable@vger.kernel.org
Fixes: 7ed8ce1 ("xen-blkfront: move negotiate_mq to cover all cases of new VBDs")
Signed-off-by: Manjunath Patil <manjunath.b.patil@oracle.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/block/xen-blkfront.c

@@ -1919,6 +1919,7 @@ static int negotiate_mq(struct blkfront_info *info)
 			      GFP_KERNEL);
 	if (!info->rinfo) {
 		xenbus_dev_fatal(info->xbdev, -ENOMEM, "allocating ring_info structure");
+		info->nr_rings = 0;
 		return -ENOMEM;
 	}