make skb_copy_datagram_msg() et.al. preserve ->msg_iter on error

Fixes the mess observed in e.g. rsync over a noisy link we'd been
seeing since last Summer.  What happens is that we copy part of
a datagram before noticing a checksum mismatch.  Datagram will be
resent, all right, but we want the next try go into the same place,
not after it...

All this family of primitives (copy/checksum and copy a datagram
into destination) is "all or nothing" sort of interface - either
we get 0 (meaning that copy had been successful) or we get an
error (and no way to tell how much had been copied before we ran
into whatever error it had been).  Make all of them leave iterator
unadvanced in case of errors - all callers must be able to cope
with that (an error might've been caught before the iterator had
been advanced), it costs very little to arrange, it's safer for
callers and actually fixes at least one bug in said callers.

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