net/act_pedit: Introduce 'add' operation

This command could be useful to inc/dec fields.

For example, to forward any TCP packet and decrease its TTL:
$ tc filter add dev enp0s9 protocol ip parent ffff: \
    flower ip_proto tcp \
    action pedit munge ip ttl add 0xff pipe \
    action mirred egress redirect dev veth0

In the example above, adding 0xff to this u8 field is actually
decreasing it by one, since the operation is masked.

Signed-off-by: Amir Vadai <amir@vadai.me>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/tc_act/tc_pedit.h

@@ -20,6 +20,7 @@ enum {
                                                                                 
 enum {
 	TCA_PEDIT_KEY_EX_HTYPE = 1,
+	TCA_PEDIT_KEY_EX_CMD = 2,
 	__TCA_PEDIT_KEY_EX_MAX
 };
 #define TCA_PEDIT_KEY_EX_MAX (__TCA_PEDIT_KEY_EX_MAX - 1)
@@ -38,6 +39,13 @@ enum pedit_header_type {
 };
 #define TCA_PEDIT_HDR_TYPE_MAX (__PEDIT_HDR_TYPE_MAX - 1)
 
+enum pedit_cmd {
+	TCA_PEDIT_KEY_EX_CMD_SET = 0,
+	TCA_PEDIT_KEY_EX_CMD_ADD = 1,
+	__PEDIT_CMD_MAX,
+};
+#define TCA_PEDIT_CMD_MAX (__PEDIT_CMD_MAX - 1)
+
 struct tc_pedit_key {
 	__u32           mask;  /* AND */
 	__u32           val;   /*XOR */