[sockbuf] Don't access fields directly, use accessor functions

[davide]

Summary

Small step towards a saner sockbuf abstraction

[davide]

Updated removing the last place where this was used. I was not certain about it before so I previously left it as-is, but it looks like in this case order doesn't matter considering we're holding the socket buffer lock across the two calls.

[davide]

I plan to commit this in two days or such, so if there are objections, please raise them.

[rwatson]

Seems sensible.

[davide]

In HEAD now.