optimize established options generation

[pabeni]

perfing MPTCP over very high speed links, shows relevant overhead in mptcp_established_options()

There are number of nested branches there, and we don't have really a fast-path for the most common options set (that is: DSS alone). Additionally we don't really leverage the constraints we have on option size, see:

https://elixir.bootlin.com/linux/latest/source/net/mptcp/options.c#L1347

We need to rewrite mptcp_established_options() carefully sorting the tests to do as little branches as possible when only the DSS is going to be added