iov_iter: track truncated size

Remember how many bytes were truncated and reverted back. Because
not reexpanded iterators don't always work well with reverting, we may
need to know that to reexpand ourselves when needed.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>