NFSd: set nfsd_serv to NULL after service destruction

In nfsd_destroy():

	if (destroy)
		svc_shutdown_net(nfsd_serv, net);
	svc_destroy(nfsd_server);

svc_shutdown_net(nfsd_serv, net) calls nfsd_last_thread(), which sets
nfsd_serv to NULL, causing a NULL dereference on the following line.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
index 7b248a2..2244222 100644
--- a/fs/nfsd/nfsd.h
+++ b/fs/nfsd/nfsd.h
@@ -82,6 +82,8 @@
 	if (destroy)
 		svc_shutdown_net(nfsd_serv, net);
 	svc_destroy(nfsd_serv);
+	if (destroy)
+		nfsd_serv = NULL;
 }
 
 #if defined(CONFIG_NFSD_V2_ACL) || defined(CONFIG_NFSD_V3_ACL)