net: dsa: warn about dsa_port and dsa_switch bit fields being non atomic

As discussed during review here:
https://patchwork.kernel.org/project/netdevbpf/patch/20220105132141.2648876-3-vladimir.oltean@nxp.com/

we should inform developers about pitfalls of concurrent access to the
boolean properties of dsa_switch and dsa_port, now that they've been
converted to bit fields. No other measure than a comment needs to be
taken, since the code paths that update these bit fields are not
concurrent with each other.
Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dsa.h

@@ -265,6 +265,10 @@ struct dsa_port {
 
 	u8			stp_state;
 
+	/* Warning: the following bit fields are not atomic, and updating them
+	 * can only be done from code paths where concurrency is not possible
+	 * (probe time or under rtnl_lock).
+	 */
 	u8			vlan_filtering:1;
 
 	/* Managed by DSA on user ports and by drivers on CPU and DSA ports */
@@ -333,6 +337,10 @@ struct dsa_switch {
 	struct dsa_switch_tree	*dst;
 	unsigned int		index;
 
+	/* Warning: the following bit fields are not atomic, and updating them
+	 * can only be done from code paths where concurrency is not possible
+	 * (probe time or under rtnl_lock).
+	 */
 	u32			setup:1;
 
 	/* Disallow bridge core from requesting different VLAN awareness