net: fix rc7's __skb_datagram_iter()

X would not start in my old 32-bit partition (and the "n"-handling looks
just as wrong on 64-bit, but for whatever reason did not show up there):
"n" must be accumulated over all pages before it's added to "offset" and
compared with "copy", immediately after the skb_frag_foreach_page() loop.

Fixes: d2d30a3 ("net: allow skb_datagram_iter to be called from any context")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Link: https://patch.msgid.link/fef352e8-b89a-da51-f8ce-04bc39ee6481@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Hugh Dickins

net/core/datagram.c

@@ -423,11 +423,12 @@ static int __skb_datagram_iter(const struct sk_buff *skb, int offset,
 			if (copy > len)
 				copy = len;
 
+			n = 0;
 			skb_frag_foreach_page(frag,
 					      skb_frag_off(frag) + offset - start,
 					      copy, p, p_off, p_len, copied) {
 				vaddr = kmap_local_page(p);
-				n = INDIRECT_CALL_1(cb, simple_copy_to_iter,
+				n += INDIRECT_CALL_1(cb, simple_copy_to_iter,
 					vaddr + p_off, p_len, data, to);
 				kunmap_local(vaddr);
 			}