detect invalid network packets

Packets must be fully contained within one page,
otherwise netback rejects them with "Cross page boundary, txp->offset: b68, size: 1460".
See https://github.com/mirage/mirage/issues/234.

This means that V1.NETWORK.write and V1.NETWORK.writev must use only
buffers that have off <= page_size and off + len <= page_size,
it is not enough that len <= page_size.

If you use the Channel module this property seems to be respected.