net: make WriteMsg{IP,UDP,Unix} return the correct number of bytes transferred
net: make WriteMsg{IP,UDP,Unix} return the correct number of bytes transferred

Fixes issue 7645

LGTM=iant, bradfitz
R=iant, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/90170046

bryanxu/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh