bridge: add support for user mode STP

This patchset based on work by Aji_Srinivas@emc.com provides allows
spanning tree to be controled from userspace.  Like hotplug, it
uses call_usermodehelper when spanning tree is enabled so there
is no visible API change. If call to start usermode STP fails
it falls back to existing kernel STP.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
index a14ac51..5e84ade 100644
--- a/net/bridge/br_netlink.c
+++ b/net/bridge/br_netlink.c
@@ -165,7 +165,7 @@
 		return -EINVAL;
 
 	/* if kernel STP is running, don't allow changes */
-	if (p->br->stp_enabled)
+	if (p->br->stp_enabled == BR_KERNEL_STP)
 		return -EBUSY;
 
 	if (!netif_running(dev) ||