iov_iter: clean csum_and_copy_...() primitives up a bit

1) kmap_atomic() is not needed here, kmap_local_page() is enough.
2) No need to make sum = csum_block_add(sum, next, off); conditional
upon next != 0 - adding 0 is a no-op as far as csum_block_add()
is concerned.

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