dhcp: Remove double htonl conversion.

After 5f0e973398 htonl was called twice: once before calling
dhcp_mbuf::compose_request and once more inside this function.
Fix this, verified on wire packets with wireshark and audited code.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>