vsock/virtio: fix MSG_PEEK ignoring skb offset when calculating bytes to copy

vsock/virtio: fix MSG_PEEK ignoring skb offset when calculating bytes to copy

`virtio_transport_stream_do_peek()` does not account for the skb offset
when computing the number of bytes to copy.

This means that, after a partial recv() that advances the offset, a peek
requesting more bytes than are available in the sk_buff causes
`skb_copy_datagram_iter()` to go past the valid payload, resulting in
a -EFAULT.

The dequeue path already handles this correctly.
Apply the same logic to the peek path.

Fixes: 0df7cd3c13e4 ("vsock/virtio/vhost: read data from non-linear skb")
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Signed-off-by: Luigi Leonardi <leonardi@redhat.com>
Link: https://patch.msgid.link/20260415-fix_peek-v4-1-8207e872759e@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>