IPMI: fix occasional oops on module unload

Olaf Kirch of SuSE tracked down a problem where module unloads of the IPMI driver would occasionally result in Oopses. He tracked that down to a variable that wasn't always initialized properly in some situations. This patch initializes that variable. Olaf sent a patch that kzalloc-ed the data, but this structure is large enough that I would perfer to not do that. Thanks Olaf!

Index: linux-2.6.17/drivers/char/ipmi/ipmi_msghandler.c
===================================================================
@@ -3470,6 +3470,7 @@ struct ipmi_recv_msg *ipmi_alloc_recv_ms
 
 	rv = kmalloc(sizeof(struct ipmi_recv_msg), GFP_ATOMIC);
 	if (rv) {
+		rv->user = NULL;
 		rv->done = free_recv_msg;
 		atomic_inc(&recv_msg_inuse_count);
 	}