csum_and_copy_to_iter(): massage into form closer to csum_and_copy_from_iter()

Namely, have off counted starting from 0 rather than from csstate->off.
To compensate we need to shift the initial value (csstate->sum) (rotate
by 8 bits, as usual for csum) and do the same after we are finished adding
the pieces up.

What we get out of that is a bit more redundancy in our variables - from
is always equal to addr + off, which will be useful several commits down
the road.

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