bridge: add missing bridge port check for offloads

This patch fixes a missing bridge port check caught by smatch.

setlink/dellink of attributes like vlans can come for a bridge device
and there is no need to offload those today. So, this patch adds a bridge
port check. (In these cases however, the BRIDGE_SELF flags will always be set
and we may not hit a problem with the current code).

smatch complaint:

The patch 68e331c785b8: "bridge: offload bridge port attributes to
switch asic if feature flag set" from Jan 29, 2015, leads to the
following Smatch complaint:

net/bridge/br_netlink.c:552 br_setlink()
error: we previously assumed 'p' could be null (see line 518)

net/bridge/br_netlink.c
517
518 if (p && protinfo) {
^
Check for NULL.

Reported-By: Dan Carpenter <[e-mail]>
Signed-off-by: Roopa Prabhu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/bridge/br_netlink.c

···
 				afspec, RTM_SETLINK);
 	}
 
-	if (!(flags & BRIDGE_FLAGS_SELF)) {
+	if (p && !(flags & BRIDGE_FLAGS_SELF)) {
 		/* set bridge attributes in hardware if supported
 		 */
 		ret_offload = netdev_switch_port_bridge_setlink(dev, nlh,
···
 		 */
 		br_ifinfo_notify(RTM_NEWLINK, p);
 
-	if (!(flags & BRIDGE_FLAGS_SELF)) {
+	if (p && !(flags & BRIDGE_FLAGS_SELF)) {
 		/* del bridge attributes in hardware
 		 */
 		ret_offload = netdev_switch_port_bridge_dellink(dev, nlh,