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 <[e-mail]>

include/net/checksum.h

···
 	return csum16_add(csum, ~addend);
 }
 
+static inline __wsum csum_shift(__wsum sum, int offset)
+{
+	/* rotate sum to align it with a 16b boundary */
+	if (offset & 1)
+		return (__force __wsum)ror32((__force u32)sum, 8);
+	return sum;
+}
+
 static inline __wsum
 csum_block_add(__wsum csum, __wsum csum2, int offset)
 {
-	u32 sum = (__force u32)csum2;
-
-	/* rotate sum to align it with a 16b boundary */
-	if (offset & 1)
-		sum = ror32(sum, 8);
-
-	return csum_add(csum, (__force __wsum)sum);
+	return csum_add(csum, csum_shift(csum2, offset));
 }
 
 static inline __wsum

lib/iov_iter.c

···
 	if (unlikely(iov_iter_is_pipe(i)))
 		return csum_and_copy_to_pipe_iter(addr, bytes, _csstate, i);
 
-	sum = csstate->csum;
-	off = csstate->off;
+	sum = csum_shift(csstate->csum, csstate->off);
+	off = 0;
 	if (unlikely(iov_iter_is_discard(i))) {
 		WARN_ON(1);	/* for now */
 		return 0;
···
 		off += v.bv_len;
 	})
 	)
-	csstate->csum = sum;
-	csstate->off = off;
+	csstate->csum = csum_shift(sum, csstate->off);
+	csstate->off += bytes;
 	return bytes;
 }
 EXPORT_SYMBOL(csum_and_copy_to_iter);