bridge: range check STP parameters

Apply restrictions on STP parameters based 802.1D 1998 standard.
   * Fixes missing locking in set path cost ioctl
   * Uses common code for both ioctl and sysfs

This is based on an earlier patch Sasikanth V but with overhaul.

Note:
1. It does NOT enforce the restriction on the relationship max_age and
   forward delay or hello time because in existing implementation these are
   set as independant operations.

2. If STP is disabled, there is no restriction on forward delay

3. No restriction on holding time because users use Linux code to act
   as hub or be sticky.

4. Although standard allow 0-255, Linux only allows 0-63 for port priority
   because more bits are reserved for port number.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_private_stp.h

@@ -16,6 +16,19 @@
 #define BPDU_TYPE_CONFIG 0
 #define BPDU_TYPE_TCN 0x80
 
+/* IEEE 802.1D-1998 timer values */
+#define BR_MIN_HELLO_TIME	(1*HZ)
+#define BR_MAX_HELLO_TIME	(10*HZ)
+
+#define BR_MIN_FORWARD_DELAY	(2*HZ)
+#define BR_MAX_FORWARD_DELAY	(30*HZ)
+
+#define BR_MIN_MAX_AGE		(6*HZ)
+#define BR_MAX_MAX_AGE		(40*HZ)
+
+#define BR_MIN_PATH_COST	1
+#define BR_MAX_PATH_COST	65535
+
 struct br_config_bpdu
 {
 	unsigned	topology_change:1;