afs: Fix vlserver record corruption

Because I made the afs_call struct share pointers to an afs_server object
and an afs_vlserver object to save space, afs_put_call() calls
afs_put_server() on afs_vlserver object (which is only meant for the
afs_server object) because it sees that call->server isn't NULL.

This means that the afs_vlserver object gets unpredictably and randomly
modified, depending on what config options are set (such as lockdep).

Fix this by getting rid of the union and having two non-overlapping
pointers in the afs_call struct.

Fixes: ffba718e9354 ("afs: Get rid of afs_call::reply[]")
Signed-off-by: David Howells <[e-mail]>

fs/afs/internal.h

···
 	struct rxrpc_call	*rxcall;	/* RxRPC call handle */
 	struct key		*key;		/* security for this call */
 	struct afs_net		*net;		/* The network namespace */
-	union {
-		struct afs_server	*server;
-		struct afs_vlserver	*vlserver;
-	};
+	struct afs_server	*server;	/* The fileserver record if fs op (pins ref) */
+	struct afs_vlserver	*vlserver;	/* The vlserver record if vl op */
 	struct afs_cb_interest	*cbi;		/* Callback interest for server used */
 	struct afs_vnode	*lvnode;	/* vnode being locked */
 	void			*request;	/* request data (first part) */