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: d2d30a376d9c ("net: allow skb_datagram_iter to be called from any context")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>