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/net/tc_act/tc_pedit.h

@@ -5,6 +5,7 @@
 
 struct tcf_pedit_key_ex {
 	enum pedit_header_type htype;
+	enum pedit_cmd cmd;
 };
 
 struct tcf_pedit {