iov_iter: make the amount already copied available to iterator callbacks

Making iterator macros keep track of the amount of data copied is pretty
easy and it has several benefits:
	1) we no longer need the mess like (from += v.iov_len) - v.iov_len
in the callbacks - initial value + total amount copied so far would do
just fine.
	2) less obviously, we no longer need to remember the initial amount
of data we wanted to copy; the loops in iterator macros are along the lines
of
	wanted = bytes;
	while (bytes) {
		copy some
		bytes -= copied
		if short copy
			break
	}
	bytes = wanted - bytes;
Replacement is
	offs = 0;
	while (bytes) {
		copy some
		offs += copied
		bytes -= copied
		if short copy
			break
	}
	bytes = offs;
That wouldn't be a win per se, but unlike the initial value of bytes, the amount
copied so far *is* useful in callbacks.
	3) in some cases (csum_and_copy_..._iter()) we already had offs manually
maintained by the callbacks.  With that change we can drop that.

	Less boilerplate and more readable code...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>