tc: pedit: add decrement operation

tc: pedit: add decrement operation

Implement a decrement operation for ttl and hoplimit.

Since this is just syntactic sugar, it goes that:

  tc filter add ... action pedit ex munge ip ttl dec ...
  tc filter add ... action pedit ex munge ip6 hoplimit dec ...

is just a more readable version of this:

  tc filter add ... action pedit ex munge ip ttl add 0xff ...
  tc filter add ... action pedit ex munge ip6 hoplimit add 0xff ...

This feature was suggested by some pseudo tc examples in Mellanox's
documentation[1], but wasn't present in neither their mlnx-iproute2
nor iproute2.

Tested with skip_sw on Mellanox ConnectX-6 Dx.

[1] https://docs.mellanox.com/pages/viewpage.action?pageId=47033989

v3:
   - Use dedicated flags argument in parse_cmd() (David Ahern)
   - Minor rewording of the man page

v2:
   - Fix whitespace issue (Stephen Hemminger)
   - Add to usage info in explain()

Signed-off-by: Asbjørn Sloth Tønnesen <asbjorn@asbjorn.st>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David Ahern <dsahern@kernel.org>