vxlan: Read jiffies once when updating FDB 'used' time

Avoid two volatile reads in the data path. Instead, read jiffies once
and only if an FDB entry was found.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20250204145549.1216254-3-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/vxlan/vxlan_core.c

@@ -434,8 +434,12 @@ static struct vxlan_fdb *vxlan_find_mac(struct vxlan_dev *vxlan,
 	struct vxlan_fdb *f;
 
 	f = __vxlan_find_mac(vxlan, mac, vni);
-	if (f && READ_ONCE(f->used) != jiffies)
-		WRITE_ONCE(f->used, jiffies);
+	if (f) {
+		unsigned long now = jiffies;
+
+		if (READ_ONCE(f->used) != now)
+			WRITE_ONCE(f->used, now);
+	}
 
 	return f;
 }