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_if.c b/net/bridge/br_if.c
index f3a2e29..cf10b8f 100644
--- a/net/bridge/br_if.c
+++ b/net/bridge/br_if.c
@@ -203,7 +203,7 @@
 	memcpy(br->group_addr, br_group_address, ETH_ALEN);
 
 	br->feature_mask = dev->features;
-	br->stp_enabled = 0;
+	br->stp_enabled = BR_NO_STP;
 	br->designated_root = br->bridge_id;
 	br->root_path_cost = 0;
 	br->root_port = 0;