powerpc/iseries: Initialise on-stack completion

get_viotape_info() declares a vio_waitevent on the stack, which
contains a completion, but never initialises the completion.

I have no idea how this ever worked, and on recent kernels it causes
an oops in handle_tape_event() when we access the non-initialised
completion.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/iseries/vio.c

@@ -474,6 +474,8 @@ static void __init get_viotape_info(struct device_node *vio_root)
 	struct vio_waitevent we;
 	int ret;
 
+	init_completion(&we.com);
+
 	ret = viopath_open(viopath_hostLp, viomajorsubtype_tape, 2);
 	if (ret) {
 		printk(KERN_WARNING "get_viotape_info: "