nFAPI/VNF: Correctly set message length when segmenting

The 5G nFAPI message length is 32bits. In particular tx_data.requests
can be longer than 64kB. When segmenting, we should correctly write the
message of the current segment (across all 32bits), because the
length would interpreted wrongly otherwise.

This fixes a bug in which tx_data.requests were discarded for 4-layer DL
MIMO on 100 MHz with this message:

    P7 unpack message length is greater than the message buffer

Further, increase the type of various (segment-related) variables to 32
bits. Currently, the maximum segment size is sxt to 65000 bytes (and in
will likely remain, because the maximum UDP size is 65536);
nevertheless, increase it in case we will ever go beyond this.

See also commit dee68e63 ("nFAPI: increase maximum segment size to
65000")