os: cut limited read to 1 GB
os: cut limited read to 1 GB

If systems actually read that much, using 2GB-1 will
result in misaligned subsequent reads. Use 1GB instead,
which will certainly keep reads aligned and which is
plenty large enough.

Update issue 7812.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/94070044

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