net: batch zerocopy_fill_skb_from_iter accounting

Instead of accounting every page range against the socket separately, do
it in batch based on the change in skb->truesize. It's also moved into
__zerocopy_sg_from_iter(), so that zerocopy_fill_skb_from_iter() is
simpler and responsible for setting frags but not the accounting.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>